Go to your used nuts and bolts bin and take a whole bunch of old used nuts and bolts, leave them on the drivers seat with a note saying "guess what I did", it will freak them out for weeks, or until you tell them. I did that to a guy I work with, suddenly his car has developed mystery noises.
Another good one is to get a box of air fresheners, the fruitiest, ones you can find, and open every single one and hide them all around the interior. Did that to a friend in high school, you could smell cherries about 5 feet away from his car, with the doors and windows shut, and it was like that for 3 or so days, it smelt like cherries for about a month afterwords, it was manageable.
The friend of mine I did the air freshener prank to got me back by setting up an O2 machine inside the Caravelle I had at the time and let it run all night. I got in and it smelt like it just rained, it threw me off, its the last thing you expect to smell inside a car, the ultimate "WTF?".