Get everyone but your target in on it and never come by his or her office twice in a row wearing the same clothes. Sanity test...
Staple every unimportant paper on their desk together.
If your target has a computer, reposition the monitor everyday.
Fill an empty white out bottle with milk and replace it with your co-workers.
Put a live lobster or any other creature in the file cabinet.
If computer has speakers turn the volume all the way up or way down depending on your mood.
Taping down the switch hook buttons on a phone gets some interesting reactions. When the mark answers the phone keeps ringing.
Program the mark's phone to forward to the office paging system.
Ask your mark, "ARE YOU GETTING FIRED? WELL, THAT'S THE RUMOR."
Does your coworker have fish in the office? Take the fish and leave a ransom note.
Pull the labeled buttons off of their phone and rearrange the order and put them back on their phone. They won't be sure of which lines is which or which connects them to the boss!
Tape your victim's telephone receiver down at top and bottom when they are away from their desk. When they come back, call them from your desk and watch them struggle to answer.
Put transparent tape over the read out of a calculator. It makes the numbers blurry.
If your boss wins some kind of prestigious award, manufacture a phony memo from the company president announcing the discontinuance of the award.
By a package of approximately 200 of those little paper bathroom cups and neatly arrange them all over the subjects desk. Then staple them all together and fill them with water. See how long it takes them to figure out how to get rid of this set-up without spilling water all over their paperwork, files, computer, etc...
Take the paper out of the copier and write "Everything written of the flip side of this paper is a lie!" Put it back into the copier mixed with regular sheets.
Buy a voice changer at Toys 'R Us and answer the phone in strange voices.
Does somebody smoke at work when they're not supposed to? Put Ambesol on the filter of their cigarettes. Watch as their lips and mouth go numb when they light up!
Get Valerian Root capsules (at health food stores) and when co-worker is away from desk, take his phone apart and open a capsule or two of Valerian Root in the mouthpiece then replace. Guaranteed to smell terrible!
At lunch, swap the worker's real food with look-a-like dog toys.
If someone is applying for a job, call them back and leave a wrong number. They go crazy for a while until you call them back apologizing.
If the drawers to the victim's desk has a board under it you can take the drawer out, take the contents out, put the drawer back in, but UPSIDE-DOWN! Then, while the upside-down drawer is partially opened, put the contents back in and close it. When the unsuspecting victim opens the drawer, all the contents fall out!
Take some cellophane and open up the glue bottle. Put the cellophane across the opening, then close the bottle. Watch the victim try to squeeze glue out. They either open it up to check, or they squeeze to hard, breaking the cellophane and spraying glue everywhere.
Tell a new worker that everyone has tomorrow off because of the boss's religious beliefs. See if he shows up the next day.