1. Wrap a car in shrink wrap
2. Cover a car in Post Its
3. Turn everything up!- audio, air, windshield wipers, turn signals ect.
4. Secure an empty foam coffee to the roof of your car to trick other drivers
5. Get magnets printed up with cheesy slogans that are bumper sticker-sized
6. Remove the tires from a car and prop 'er up on cinder blocks
7. Fill the car up with balls or balloons so that, upon opening the door, they roll out
8. Line the trunk of a car in plastic, and then fill the trunk with water. Sealing plastics and drop cloths work best for this, you don't damage the car
9. Hit and Run! Leave a note on someone's car, explaining that you're terribly sorry and you'll pay for the damages. This prank only works if you didn't actually hit them
10. Buy a cheap window, then break it. Sweep up the bits and take to your targets car. Roll down the window and spread the glass on the seat and the ground around the window.