My party is about to go through a large area of forests, hills, rivers, and fields, and I wanted to spice it up with some random encounters. I already have some battles prepared, so preferably non-combat encounters work. Any ideas?

I have 3 so far:

Field:

A white Harengon nearly runs into your party - he apologises profusely before running away, in the distance, you can distinctly hear him say 'I'm late'...

You stumble across a fleshy mound, about 1m across and 3m long and ...is that a fingernail?

It has to be a ship, it's definitely a ship - but the ocean is miles away... 

The crops here have been flattened, forming some kind of circular pattern.

As the wind whistles through the cornstalks, you're positive it calls the name 'Schubert'...

Woah, you almost stepped on it - some sort of doll house, wait... are they moving?

The party meets a small merchant caravan (three or four wagons).  The merchant is delivering a wagon load of live monkeys to a local ruler.  One monkey is albino and especially rare, and is actually going to be delivered to a local mage.   As the party meets the caravan the wagon load of monkeys tips over (due to rough road, bad driving, sabotage, whatever fits your circumstance best).

The merchant tells your players he'll pay one silver for every monkey captured alive and ten gold for the albino.

As the party emerges from the darkened forest, following the old dirt road toward the simple farming village a mile or so hence, they see ahead of them a bandit standing in the middle of the road. The bandit is small, the size of a human child of maybe 8 or 10 years, is barefoot and is wearing simple farmer's overalls. The bandit appears to be holding a short sword in one hand and a round green object in the other.

Nearer, the party notices two more bandits off the left side of the road. One is a short chubby boy of maybe 10 years, wearing a rough flaxen tunic and holding a spear made of a small tree branch. The other is a redheaded teenage girl in dirty overalls, with what looks to be green paint or ink on her hands and a rusty hand axe in one hand. In front of these two there lay propped up by a log three figures that very vaguely resemble goblins, but are clearly just dirty clothes stuffed with straw. Atop two of these "goblins" is a burlap sack rounded by what is probably a small pumpkin inside it, and painted green, with two X's for eyes and a frown drawn on with charcoal. 

As the party approaches, the bandit holding the roughly-whittled short sword holds aloft the green "head" of the third goblin and addresses the party in a squeaky voice of false confidence: "Well met, uh... weary trav'lers. We are um... brave knites who have, thorted these nasty gob'ins... (he lowers the raised goblin head because it's clearly making his arm tired)... makin' the road safe for y'all. Any, uhh, coin you wish to, umm... (he glances at the redheaded girl who loudly whispers the word "contribute")... contribute to our nob'l cause would be greatly apprec'ated."  

If the party donates anything - even a single copper coin, the boy will collect it up and thank them politely. Donation of a silver coin will prompt the boy to beam with joy and yell to his compatriots, "Holy shit! Guys! Silver! Hey, thanks folks! Wow!" 

If the party donates nothing, then within a few hours word of their stinginess will have spread through the nearby village and many of the folks in that tight-knit community will give the players the cold shoulder and maybe even charge them a higher than usual rate for whatever they try to buy. 

For a rest stop, or upon encountering a pond or stream during travel:

A swarm of eight Stirge.

One of my players once made an offhand comment about the pool of water they arrived at being so clear and clean that they knew they were playing an RPG because there were no mosquitos...my mind instantly flashed to Stirge who attacked as they all laid down to rest. (Was that too mean do you think?)

For when the party enter an area of the forest that is a little less trodden...a group of Dryad emerge from the trees demanding that they turn back now lest they enter the sacred area of the forest that they protect because of reasons. It literally doesn't matter the reason, could be the place where new dryad are born, could be the place they first emerged from the Feywild (or your equivelent) into this world, could be a magical spring. Literally the important point is that the Dryad protect it and will unleash nasty things to ensure humanoids stay out.

For the encounter itself, I'd have several Dryads if it's a low level party. If a higher level party I'd throw in some Treants and/or Treant Spalings. There's also the option of awakened trees.

As the party continue forward they notice the air starts to get thick. They start sneezing because they've trodden on ground laden with fungal spores. Have players make a CON Saving Throw, at a DC 12. On a failed save the character begins to hallucinate and is incapacitated while they do so. 

Should they ignore this small detail and continue on forward, Myconids appear. A colony inhabit this part of the forest including a Myconid Sovereign. three Myconid Adults, and several Myconid Sprouts. These are Variant Myconids who are able to survive in the singlight, or at least under the forest canopy (ignore the Sun Sickness in their stat blocks).

Initially, the Myconids are hostile to the party as they are in the process of releasing spores and growing new Sprouts to rebuild their fungal colony. If the players try to talk rather than fight, the Myconids reveal that there is group of Lizardfolk who have begun to make this part of the Forest their home and have embarked on a campaign to obliterate the Myconids. It is for this reason the Myconids have laid down Hallucination Spores in this area, as a defence mechanism.

One way to generate encounters is to create a list of creatures in the area and some generic activities like fighting, traveling, resting etc... Then you can just roll a creature and an activity and get a bunch of encounters. For some activities you might roll another like fighting you'd roll for what they are fighting.   For creatures  you only really need one or two of each, a legendary creatures, npcs and factions. You can give the factions some relationships too like ally, enemy or neutral. 

You can also expand on what you have already a bit. 

So you have a griffon so you can throw in 

• The griffons nest 
• Hunters looking for the griffon/ it's nest 
• food for the griffon like wild horses

There's grung in the area so you might have. 

• A grung trap like a snare possibly with something stuck in it
• A grung hunting party
• A grung hermit of some kind like a spell caster or shaman

These all have fairly flexible dynamics and can interact with each other. For example the hunters or shaman may want information on the griffon or it's nest. The shaman or druid may be able to provide healing for the griffon if the party can't or may reward them for saving the creature. So you've created a little random quandary there. 

I made an adventure with a bunch of random encounters in a forest if it helps.

While the characters are walking through the forest or field, have them make a perception check. If they fail, then they stumble into a mushroom ring and immediately begin to dance uncontrollably. It is quite a large ring and the rest of the party members have to find a way to rescue the helpless dancing victim. The ring should be at least 20 ft in diameter though it could be bigger. If anyone enters, they also start dancing, if the ring of mushrooms are disturbed or damaged, then a couple fay creatures show up yelling at whoever harmed their ring. They may attack depending on the level of the damage to the ring and depending on what type of faeries they are. These faeries should also only speak in rhyme. The fay may require them to perform some sort of side quest to appease them. Or they might require them to be paid with their happiest childhood memory, their eye color, their first lost tooth, a tear of joy, or some other similar payment.   

Or, have them come across a feast set out on a large table. Around the table are seated a group of awakened woodland critters like badgers, squirrels, rabbits, turtles, mice, etc. Maybe the meal is a Hero's Feast. The animals were all awakened by a druid who lives nearby. The druid has been awakening all sorts of animals in order to start his own forest community and to protect the woods. After polite conversation with the animals, which could involve the players roleplaying, the animals giving them valuable lore or information, or could just be a way to relax the party, the animals may give directions to this druid who could provide them with lodgings, or aid in some strange nature-themed form. The druid might try to convince the players to go and find a large amount of valuables that he would require to continue to awaken animals. He could be a lonely old fellow who wishes to awaken the whole forest in order to provide him with company and to protect the place.

You see a scarecrow in a field. As you look at it, it moves. But there is no wind. Is that an actual person tied to a stake...?

A big flat slab of rock, which is far too big to move. It's warm, there's no explanation of why. Makes a nice place to make camp.

A meadow in which stands a terrified statue of an elf, who looks like he's screaming. He was petrified by a Cockatrice, and is about due to de-petrify, at which point he keeps screaming and just outright flees. Good foreshadowing of a cockatrice encounter as well.

A tree which is actually growing out of a big hole, its top only inchas above the surrounding ground. It seems this type of tree grows downwards.

A corn maze. After entering and getting slightly lost, the sun starts to set, and they each take on a different coloured glow as plants sprout large, glowing white fruit at 5-foot intervals. And that's when they hear this noise on the wind, growing closer... "Wakka-wakka-wakka-wakka...".

A standing stone with something scratched into it in Draconic. If they can read Draconic, it says "Do Not Remove".

A stone archway. Anyone who walks through it travels d6*10 seconds into the future. Just narrate that they disappear, and don't return to them until the time has passed, at which point they emerge from behind the arch.

A huge flock of giant birds fly overhead. Guess they've eaten something that disagrees with them, make a dexterity saving throw!

A tree which is scared of people. It shakes violently, as though there were hundreds of invisible things in the branches. This is counterproductive, as it makes people get closer to investigate. If spoken to with speak with plants, it will just plead for them to not hurt it in a really pitiful voice.

A curious awakened shrub which follows them. It only becomes obvous if seen or when they get to a field.

A group of lumberjacks who are felling a large tree.

A group of dwarven lumberjacks which are using a huge machine to cut down trees, and are being fended off by Fairies.

An iron golem with an axe which seems to be frozen mid-swing. The magic has gone out - it's just a steel statue now.

A friendly tiefling sat in a tree playing a fiddle.

A huge, burly half-orc tending a warren of rabbits. If combat ensues, the rabbits swarm.

A field where entering it shrinks you. The grass grows longer, and thicker, until it towers over you. Soon, you face a great beast - a Hawk. As you leave the field, you return to normal size. If someone is picked up by the hawk, it flies to a tree - and the character grows as it does so, until the hawk simply releases them. Good for a tense moment with very little actual consequence - a little slashing damage from the talons as they're picked up. 

A halfling runs past dressed as a fox. Moments later, a troup of halflings in red, riding baying hounds, thunders across the field after the fox. If "saved", they will be annoyed, because this is just hunting practice for the dogs, and the "fox" isn't in danger!

A forgotten pitfall trap containing a starving and scared animal

A Bunnycorn or faerie dragon caught in a snare. They are grateful if you release them.

A once-evil spirit trapped in an ancient tree. if cut out, will take the form of a changeling, and will express uncertainty over what to do, because they're a changed spirit, and therefore feel they lack purpose. They're only good at evil stuff. The party can offer guidance on what the changeling can do with ther freedom.

Forest... go classic out of Arthurian legend the White stag.  or you could with the characters accidentally finding their way into the realm of the Fey

Quote from ArmedCoward >>

The party meets a small merchant caravan (three or four wagons).  The merchant is delivering a wagon load of live monkeys to a local ruler.  One monkey is albino and especially rare, and is actually going to be delivered to a local mage.   As the party meets the caravan the wagon load of monkeys tips over (due to rough road, bad driving, sabotage, whatever fits your circumstance best).

The merchant tells your players he'll pay one silver for every monkey captured alive and ten gold for the albino.

Then I'd make the Albino monkey a polymorphed or otherwise cursed adventurer who can talk to add a moral dilemma.  And with exhaustive investigation or seeing through the monkey's deception, the crew finds out the person was rather despicable and probably deserved their punishment.  Maybe as a merchant he ran over the child of a hedge witch and crippled or killed them. 

Thanks for all the ideas! I love the Albino monkey idea and ThorukDuckSlayer's long list.

Duffinsmertz  kobald artifcer traveling merchant who sometimes  has inators aka magic items.  My players find this guy funny  and amusing.