Easy fun for Monday.  Go to the Mozmonkey Blog and study up on the "Stinging Keyboard" prank.

Our suggested method of implementation:

When you're at work, check out your best friend's (you'd never do this to someone you weren't best friends with, right?) keyboard. 

Then go out and buy one.

Assemble hack in the privacy of your own home/lair/mansion/cave.

Sneak in on the weekend or after hours and replace their keyboard.

Extra bonus points, two words: Hidden Camera.