When one of your fellow workers goes on vacation, try this funny prank:
Promise a homeless guy ten bucks to come and sit in the vacationer's cube on the day he- the vacationer- is supposed to come back to work. The vacationer will come into the office, see the homeless guy in his space, and think, "Oh man, I been replaced by a homeless guy! I am such a loser!" (which he is, by the way), and then he will be so distraught he'll run screaming from the office. Later, when he's just about to take his own life, you burst through the front door of his apartment, pull his head out of the oven and say, "We was just kidding you, man!" Then after you're all done laughing about the funny joke, you have the homeless guy come in and you tell your co-worker to give the homeless guy the ten bucks you promised him.