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.
Step 3: Full Blast!
Take the keys to your car-loving friend's auto, and turn the radio up full blast. Bonus points for a Tejano station, especially if your friend likes to jam to rap and rock. Turn the air conditioner on cold and full blast (hence, the name). Also turn the front and back windshield wipers on, and maybe the hazards or brights for good measure. Take it to the limit by adjusting the steering wheel and seat base all the way up and forward, making it hard for the driver to enter the car.
If you also put Vaseline on the steering wheel and gearshift, they'll think that's the end of the prank. The double whammy is great, as they'll already be frustrated (or laughing) when they turn on the car.
Step 4: Coffee Cup!
Secure a foam cup of coffee (empty obviously) to the roof of your car, and take a spin, ostentatiously making sharp turns on roads where plenty of people are able to see. Make certain that everyone's watching you and wondering how your master handling gets the cup to stay put, especially while you do donuts in a parking lot.
Bonus points for doing this with a car seat and a doll while speeding down a highway with your flashers on. Double bonus points? Not getting pulled over.
Step 5: Sticker Price!
Get magnets printed up with cheesy slogans that are bumper sticker-sized. Place them all over the car in question, and wait for your reward.
Bonus points if you get political. Minus points if you get too crass.
Step 6: Cindered!
Bonus points for doing this on the day that your victim has a big meeting, or is really nervous about any "big deal" event for which they're going to have to make a drive. Double bonus points if you hire a NASCAR pit crew to replace the tires in three minutes flat, so that your car enthusiast loved one won't be late. Minus points if you get them fired for not showing up to that board meeting on time.
Step 7: Balls to the Wall!
This also works great with styrofoam peanuts — the most economical, but least environmental of the bunch.
Bonus points if you find a way to incorporate a gaggle of white turtledoves to also fly out of the car, gloriously punctuating your prank. Minus points if you kill the doves.
Step 8: Junk in the Trunk
Be sure that you have an exit strategy, because remember: cars don't flush.
Step 9: Hit and Run!
Leave a long note on someone's car, explaining that you're terribly sorry and you'll pay for the damages. This prank only works, by the way, if you didn't actually hit them. An alternate message could read "I just got seen hitting your car, and I'm pretending to leave my info."
Step 10: Broken Window!
Then run and tell your victim that you think something happened to their car.