[BECCALICIOUS!]

If you don't want a scary prank, my favorite is putting salt on someone's toothbrush... you can't see it and it tastes NASTY!

I also put a piece of tape over the laser part of my dad's computer mouse so that when he moved it, the cursor wouldn't move so it'd look like the computer was frozen... my dad's an engineer and it took him nearly 3 hours and he almost called a repair shop!