Open a word document, smear whiteout on the screen, and take a photograph of the monitor with enough of his desk showing to make it clear who's desk it is.
Take the ball out of his mouse.
Whoopee cushion, oldie but goodie.
Buy flowers and large metallic balloon that says "woohoo 40 years old!" (or 50, whatever the next big jump for this guy would be) and have them delivered to his desk/cubicle.
Paint his name on a parking spot right near the front of the building, preferrably in a better spot than his boss.
Bend all of his paperclips open just enough so they don't grip paper anymore.
Every day, or at least every Friday, leave an anonymous foam cup with about 1/2 inches of coffee in it on his desk when you leave work. After a while, leave one on his bosses desk.
Have empty envelopes sent to him at work.
If you feel creative, send a series of them with an official sort of markings. After about half a dozen of these, send a blank envelope marked "notice of unpaid debt" in large letters.
Delete all his internet explorer cookies and reset the password cache.
Delete all .pwl files on his computer.
Password encrypt his .pst file if he leaves his email open. Make the password his last name so when he complains about not knowing the password, you first gather a few people around to "help", then casually suggest he try something easy to remember, like his last name. Roll your eyes when he protests that he didn't do it.
Password encrypt someone elses .pst file with your opponent's last name, then suggest that the innocent victim also try this guy's last name since he didn't remember messing with his own file, maybe he messed with someone elses too.
Change various shortcuts on his desktop to point at command.com.
Change his default printer to the one in the CEO's office, or at least another printer that would be publically embarassing to send a dozen documents to while wondering why they're not printing where they usually do.
Take someone's phone off the hook, and then set your enemy's phone into busy-wait callback mode to that extension. When the off-the-hook phone is placed back on the hook, they both ring and connect to each other with the usual confusing results. Why are you calling me? I didn't call you, you called me!
Pretend to trip on something every time you walk past his desk or cubicle.
Transfer supplies from the cleaners closet to his desk drawers or cubicle. Point people in his direction when they're looking for more toilet paper, paper towels, or other cleaning supplies.
Remove the erasers from all his pencils.
Replace his whiteboard pens with permanent, permanent pens with waterbased, etc etc.
Leave the company prospectus for your companies biggest competitor on his desk in plain sight while he's away. Put prominent yellow stickies on certain pages with notes like "mention this during interview".
Place 2 polarized anti-glare filters over his screen, rotated 90 degrees from each other.
If he uses a laser pointer or any other device during presentations, ensure that the batteries are dead before he gives his next presentation.
Sprinkle sawdust under his desk or chair and claim you saw a mouse under his desk.
Use all of his kleenex.
Replace his yellow post-its with pink ones.
Switch all the caps around on his highlighters or colored pens.
