Silly lore in your campaigns.

And another, found while I was doing proofreading, lol.

Swamp Trolls can summon the very roots and vines to do their bidding, and at least one is said to cause burns on those who fear it when touched.

I need less free time.

I have a lot of silly lore in my book I'm making for the eccentric artificer in my world, Gizmo Grayling.

He invented a crossbow which fire bolts which can hunt down a creature it has the blood of, so that he could give his stubborn nephew a flu shot.

He built the Hammer of Probability, which symultaneously jumps all the molecules in the targets armour 5ft. to the left, using a strange book he once traded from a stranger in exchange for one of his deluxe multipurpose towels. Using the same book, he created the S. E. P Medallion, which makes you functionally invisible.

He invented a bow which magically launches a projection of your fist for a barbarian who was frustrated that she couldn't punch things that are far away.

He invented a dagger called The Peeler, which was the result of an almost-failed experiment to make the sharpest blade possible. Having destroyed everything else in his shop, he finally succeeded - with a potato peeler.

He has in his shop the Cape of Arra the Klutz, a goblin reknowned for his appalling swordplay and abysmal luck. The cape causes people to laugh if you roll a natural 1, as you get tangled in it, trip on it, get it snagged in a tree, etc., and it automatically resizes to be just-too-long.

He invented the Liquid Safety Net after a run-in with a sadistic dragon who found out he had a fear of heights.

Yeah, Gizmo is a very fun character to bring into the world!

In my games, it's possible to use a small amount of uranium as an additional material component for any spell that deals fire damage.

Of course, this usually means that you don't cast the spell more than once, as it apparently angers an enigmatic deity known as Norad.

In my home-brew world, the god making abilities of the Kuo-toa comes from the innate magic residing in their tongues. So people will sometimes collect their tongues to try and use that power for themselves.

Another one is an inadvertently eco-friendly alliance of evil, consisting of a Necromancer, Devil, Vampire, and Mind Flayer. They submit their victims through a purification ritual where the body is divided amongst the four leaders.

The soul is extracted and given to the Devil
The blood is drawn and given to the Vampire
The brain is removed and given to the Mind Flayer
The flesh is burned off and the bones given to the Necromancer

In the world of Tanmar, gnolls have a genetic fear of rabbits, and earth genasi eat rocks. Also, the god of modrons is named Dennis.

In my world, Athos, there was a cow that was able to take out a party of adventurers. The party in question were the players at my table. One was a pyromaniac, one stole from the dead, one was a plain pickpocket, and one was a dragonborn paladin. all of them somehow rolled very low and all got destroyed by a cow.

In my world we (my players and I, collectively) have established that Halflings invented the game of golf, Goblins invented bagpipes, the Draconic language sounds like French, and horses are actually intelligent but benign extraplanar creatures that are just visiting the human world.

There is an elvish culture that is known for their romance. So much so that every word in their language sounds like it could be a kiss.

in one of my campaigns there's a random child who we have to keep alive for a journey across a continent to "find their parents" and im concerned the campaign will never end

Doing some minor editing still, and came across this reference to a culture hero:

Marlow Lovecraft: One of the most famous of the Reeves, he was said to have brought to justice more men than any lawman before or since, and that they always confessed. Some called him the Eldritch Horror. He just called it an honest day’s work.

Seriously, my place is chock full of them.

Most merchants in my homebrew continent pay homage to the god of trade 'Runke' by keeping her holy symbol in display in their storefronts. Merchants that don't tend to perform about 2% worse in terms of earnings. 

In my newest campaign, gnomes are given a cow at birth which they keep with them always. They bath together, eat together, sleep together, and even go to school together.

This is because Gnomish wedding traditions dictate that a couple must wash each other's feet with fresh milk from the other's cow, slaughter their cows together, and then cook and eat the livestock -- symbolizing the death of childhood innocence -- before they can be considered married.

For my world, It's probably the comet that came eons ago and turned some of the animals into animalfolk.

While Emperor Hallel, ruler of Vanalesse, being a monk isn’t in of itself silly, I do get a giggle at the thought of the party about to encounter him, expecting perhaps that he’ll bring out the imperial line’s artifact weaponry or the most advanced spell, and instead he channels his inner Hoarah Loux and Senator Armstrong and breaks out a bunch of wrestling moves on them.

In one of my upcoming campaigns a vampire will that is allergic to water not just holy water and flowing water but all water and lives in a cave under the sea