PS. People love their cars....if you damage them the least you can expect is some serious revenge on you and at the worst an a$$ kicking, so be careful who you prank!
Step 1: Wrapp'd!
Just gain access to the car (you don't need the keys for this one) late at night, and proceed to wrap at your leisure. It's like wrapping a house, only with less clean up. And you should use something more sinister than toilet paper. Be liberal in your application of the Saran wrap — the harder it is for them to clean up, the longer you have to point and laugh.
Step 2: Post-It! (on their car!)
Bonus points for fun shapes, or writing a message with differently colored Post-Its. Minus points for using Silly String instead of stickies — you're (hopefully) not in fourth grade anymore.