If you have access to someone's vehicle:

1) Talcum powder in the heating vents... turn heater settings on high w/ the car turned off... when they start the car... POOF!

or 2) For someone's birthday... fill the floorboard and seats full of balloons filled with shaving cream. On top of those... fill the car to the ceiling with balloons filled with air. Tape a birthday card or note on the window with a needle saying "you might need this". Once they get a little over zealous popping the balloons full of air... they'll start popping the balloons filled with the shaving cream.