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.