what are some little ideas i can use for one-shot-quests that i give my players in a tavern

Dozens of options.

Populate a small dungeon, which is a family crypt taken over by undead, goblins, orcs, demons, whatever you feel.  Job is to clear it.
Rescue a child stolen by kobolds, goblins, orcs, whatever.
Recover an item stolen by bandits, = Track and find bandits, then get item.
Gather herbs and plants an alchemist needs for a potion to save the local lord's wife.

There are loads of ideas you can draw on, and length will be determined on how much you put into it.

Join the polar express and guard it from packs of winter wolves. But all is not as it seems. The polar express is being run by demons in human skin that are going to kill and devour all the children upon the conclusion of the trip. The players must figure this out from whatever clues you’ve left them, and try to survive as the train turns into a death trap.

There seems t obe a minor earthquake, and everything outside goes dark. Upon investigating, it seems to be night time. But there's also no outside - just space.

Instant segue into a spelljammer game - but the tavern is the ship. Turns out a wizard got drunk and did this as a spell - and he can only keep concentration if he's drinking. They have until the tavern runs out of alcohol to work out how to get home, or everyone in the tavern will be adrift in space without any oxygen.

I have 4 I can think of for little kids and adults with a sense of humor. 1.kobalt bandits robbed a shipment help get it back turns out kobalts are working for a dragon.2.awakened teddy bear asks the party to find there owner turns out its a old wizard who rewards party with spell scrolls .3.a ghost is hunting  a house  plot twist its old man Jininkins 4.theres a problem  in the sewers workers go in but don't  come out find out what's going on than have a gelatinous  cube be in the sewer.

So, I have a post-it note on my DM screen it reads as follows:

Quest Prompts

'Find my/our/the _________'
'Protect me/my/our/the ________'
'Kill my/our/their/the ________'
'Investigate the _________'
'Explore ___________'
'Gather __________'
'Deliver __________'
'Pursue/Capture/Hunt __________'

This legitmately is what I look at when I need to generate a quest on the spur of the moment. Roll a d8 or decide thematically. The blanks can be almost anything depending on who the player characters are talking to. Is it the local blacksmith? Then it could be a hammer, daughter, or an animal whose hide is particularly rare or strong. Is it the local herbalist? Maybe it's a customer she doesn't quite trust and regrets selling nightshade extract. Literally all quests boil down to these eight things. The blanks are mcguffins. 

Depending on how you explain the quest players rarely notice in truth. Just don't explain it always as 'kill the this.......'

Let's say you look at the list and decide the blacksmith wants you to kill a bear. Then you ask 'why does the blacksmith need this done?' Maybe he needs the fur. So, the players take it from there. Their options are more than you might think. Here are some that my parties have come up with before now.

1. Kill the blacksmith because using the fur of innocent animals is wrong (I've had this happen many times and leads to a lot of interesting subsequent encounters.
2. Go, hunt, kill, and skin a bear.
3. Wait until nightfall, break into the smithy steal his precious metals and money (or the rewards you decided on).
4. Have the druid go and talk to the local bears then convinced them to attack the town (absolute hilarity ensued).
5. Tell the blacksmith to get lost and go and speak to someone else in town for something else to do.

Either way, using those prompts then asking 'why' gives you a fairly easy way to generate quests over and over again.