I like this one. Easy, simple, relatively harmless, and vicious....

 

why not wrap it with saran wrap, that's always fun to try to get off

 

If you're every driving with them, wait for a stop sign/light (only do this with minimal/no traffic) Distract him - "Hey, what's that blah blah over there!?" Then simply pull out his car keys and throw them out the window . . . If you want to screw him (and yourself) even more, you can lock the door when he goes to get them.

Put something (like a racquettball) in their intake so that the car will start and drive slow, but will be bogged at higher rpm's.

 

You know this is from 6 months ago right?

 

i hit a (unrecognizable) animal late one night, the carcass got caught up on the mufflers and cooked, the smell was horrible for weeks. I tried everything to rid the foulest smell imaginable. I even pressure washed the undercarriage, it took weeks for it to dissipate.

i know this post was from a while back, but for revenge id go with crickets or fleas. you would never get rid of all of them. no real damage to the car, just complete nuisance and those little pesty flea bites around the ankles back of the legs (or even up the shorts) would serve them right for their prank.