Talk loudly while on the phone if it's a business call so they ALL know you're working.
Put a piece of clear tape over the bottom of their mouse to stop the ball from moving. See how long it takes them to figure it out.
Hum songs like 'Mary had a little lamb' until they subconsciously pick it up and start humming too.
Spike the coffee with salt, and mix in some salt in with the sugar.
Collect dead things like flies in a jar and keep it on your desk. It will be sure to gross out someone.
Insist on being addressed by your job title instead of your name.
When a coworker approaches, pick up the phone and start talking. If they try to talk to you anyway, roll your eyes and say into the phone, "Can you hold on for a moment, I have a really rude coworker trying to talk to me when I'm clearly on the phone." Then stare at the coworker annoyed and demand to be told what was so important that you had to put someone on hold for.
Don't wear deodorant so you stink, or drench yourself in perfume/aftershave.
Prank call your coworkers by phoning and hanging up. This works best when they are across the room and have to run for the phone.
Leave plastic figurines on their desk when they're not looking.
If they bump into you or have any sort of contact with you, glare at them and mutter something about sexual harassment in the workplace.
Send them memos every hour to keep them posted on what you're doing, or what they should be doing.
Respond to everything they say with a question, and when possible, work in 'Is this good for the company?'
Have your email set to emit a stupid sound when new messages arrive. Crank up the sound, go to another part of the office where you won't be seen, and start emailing yourself.
Use the intercom/paging system to ask a coworker to contact you instead of just phoning them directly, or turning your chair to just talk to them. (This works really well if their desk is beside yours.)
Ask someone to do something for you, and tell them how to do it. Then when you return, say they did it wrong and do it a completely different way then you had told them.
Mess up the settings on their monitor.
Replace all their pens with empty or leaking pens.
Start wearing really odd clothing and see how long it takes for someone to say something.
If you don't have uniforms, begin a petition to bring them into the company. Suggest really awful uniforms and see how many people will actually agree to them. (Create sketches to show them the suggested new uniforms.)
Type as loudly as you can and sigh loudly when you need to use the backspace key.
Sing badly and off key but do it only around specific coworkers, and only when THEY can hear you.
Think up new forms for them to complete and revise them each week.
Insist that everything be submitted to you in writing on the proper forms, even if they're just asking you how you are today. (Create 'greeting' forms etc that they must use.)
Get new procedures passed at meetings and then, after two weeks, find a new way to do it but don't tell anyone.
When someone uses the old system say 'We don't do it that way anymore. Weren't you at the meeting?'
Lose their expense forms or repeatedly make errors on their expenses/pay.
Create an official sign in/out sheet for the bathrooms. See if anyone uses them.
Take awful shits in the bathroom and leave it to stink up the place. If someone complains, burst into tears and claim you have 'a problem' and you're sensitive about it.
Adopt accents and talk with them for a few weeks at a time. Fun accents to try, Russian, German, British, French. If possible, mix them all together.
Do their job for them when they're not looking, and then give them shit for doing a lousy job.
Talk to yourself when others can overhear you but make it all business jargon.
Mimic them whenever they say anything.
Repeat everything they say as a question. ("Have you seen these reports?" "Have I seen those reports?" "I gave them to you last week." "You gave them to me last week?")
Link all their paperclips together.
Bend all their staples so they don't work right in the stapler.
Collect the paper bits from the hole-puncher and throw it around their desk.
Have really stinky food for lunch.
Sniff them. Try to determine what they smell like, and compare it to the way they smelt the last time you sniffed them.
Smile at them non-stop, especially on days where they're feeling stressed.
Send them messages thanking them for being a special person, and that you truly appreciate all the work they do.
Wear completely bizarre outfits in the brightest colors you can possibly find. If someone comments, claim your handicapped neice/nephew made it for you and that you think they're an awful person to be making fun of handicapped people.