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.