Asklemmy

A balloon filled with helium tied to the handle. (How did that "fall out"?)

A gallium coin (if it's cold outside then it'll stay solid and then melt when they put it in their pocket).

An opened (but unused) bandaid. The biggest one you can find. Stick it to the handle so it flaps around and they have to choose between touching the gauze (it's clean, but they don't know that) or the sticky part to pull it off.

A household smoke detector. Use a piece of string to tie it to the handle.

Baby shoes. Again, tie them to the handle.

7 worms in a bag. They're lucky.

Whenever you go into a gas station, buy a random keychain and put that on there. Watch your friend start drowning in keychains.

Christmas lights. Just all of them. All the Christmas lights all over the car. But make sure to thread them through the driver-side handle and include your "I think you dropped this" note.

One serving of peanut butter

• one penny
• marbles (something something lost their marbles)
• a lego brick
• origami crane
• notes saying ridiculous stuff like "remember to water your car before you leave" or "this car belongs to [your name and address]"
• a very embarrassing mini figure like maybe barbie, winx club, things like that

A screw, bolt, nut, etc. that could feasibly have come from the car itself

A very tacky car freshener. Some are dirt cheap, the cardboard ones I'm talking about. Pick a design that is silly or ugly for them... Or maybe a fragrance that you know they're not a fan of

A dozen wasps.

Level of aliveness is irrelevant.

First 5 things off the top of my dome:

a lego figure, woodwind reed, mahjong tile, plastic spider ring, a tiny canister of tic-tacs.

A couple spoonfuls of spaghettios. You know, as a snack.

A pocket dimension that warps back around to the car itself.

"I think you dropped this" and its just a big dollop of Vaseline right on the handle

Dog toys

Rubber chicken. Though don't do that to Jair Bolsonaro car, then it would be NSFW object.

An old shoe. Just one. You can even tie it on by the laces.

A half of a sandwich with a few bites taken out of it.

Roll up a 'trump 2024' poster. Pikachu keychain. Pom Poms' from the dollar store. Tie a pool noodle to the driver side doors (with a bow knot that can be undone easily). A really long striped stocking, hot pink or some neon color.

Snack bar wrappers. Really weird ones that only really little kids love.

A partially eaten apple

nerf dart

A cracked egg, or the lid of the pack of sugar you put in their gas tank. A weapon that was used in a murder. I guess it just depends how much you hate them

A banana.

Underwear, either gender. Though you said no value, so uh, dryer lint? Clothes pins? Dog biscuits? A dog poop bag with pudding in it?

A toothbrush.

A roll of Pennies?

Make a plaster mould of the exact shape of the inside of the handle so it's really annoying to take out