From my page-a-day calendar:
Four Prank Ideas for April 1st
Paste a small Post-it note under someones computer mouse, covering the track ball and making the mouse inoperable. Remember to write April Fools! on the note.
Some computer keyboards have removable keys. Find one that does and mix up the letters. (Be careful not to permanently damage it, or youll be the fool.)
Smear honey on your victims car door handle, or if at school, on their locker handle. If youre really bold, smear some on the toilet seat.
Two words: Oreos, toothpaste.