If you're a sysadmin, you DON'T want to fitz with the computer systems at ALL. I've heard of too many pranks that have turned real ugly.

How about building a tall stack of cardboard boxes that are made to look like servers, carry them around and pretend to drop them in front of important people (like your CEO). Okay... too much?

Smoking monitors using pretend smoke?

Bullet holes/cracks in monitors using those stickers?
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There is still time to order the "annoy-a-tron" from Thinkgeek: http://www.thinkgeek.com/gadgets/electronic/b278/
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I have to agree with @drmatt about being very, very careful about abusing your powers.

However, if you KNOW you'll get away with it, then set the default homepage for your intranet to http://yougotrickrolled.com/
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You could use netsend to issue out the "Pink Slip Virus" on co-workers...

Depending on your video card/drivers, you could go around pressing CTRL-ALT-DOWNARROW on everyone's system to flip their screen upside down...

Or...the one I'm planning. Get a small soundbox and have it play the sound of a cat meowing...just once every 5-10 minutes...place it inside the ceiling tiles...they'll go nuts looking for it.
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Find the coworker with the most cluttered desktop. Take a screen capture, set that as the wallpaper, then grab all the contents of the desktop and put them in a clean folder (this is your backup).

It's a mild prank, and you will get a good laugh out of it. No harm is done, the contents of the desktop are safe.
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This evil plot could only work some places, but I deploy it thusly. YMMV.

Create a fake email address from human resources, or invent a manager nobody knows about if you don't have an HR. More outrageous the better. Name?

I suggest Craig Lemonjello, pronounced "le mon gel o", btw,

and email the entire company that April 1 is wacky tie day or crazy hat day or something really dumb. Make sure EVERYONE gets it, and kind of imply it's not really recommended as much as mandatory.

Then, come april 1, you can see who in the company will take orders from non existent people, departments or managers.

This is the real trick. This is merely cover that gives you the opportunity to have a laugh and get an easy excuse for having some fun hacking if busted. What the prank really does is allow you to find marks to exploit! Is your boss not only a jerk but a moron? You'll know by his wide, speckled tie! Is the guy in the next cube gullible enough to do your daily menial tasks? Get ready to find out what a ten gallon foam hat can really signify! He'll be neck deep in TPS reports by next week.

oh yah, you probably shouldn't take any of my "advice". But seriously.. don't mess with your job. Mess around AT your job! Put a rubber rat in the fridge or tell your buddy his car is being towed or something.
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Agreed. This is a frightening idea unless you work at a small firm. Some ideas are, change the Userinit key to create an automatic logon - logoff loop. Or maybe change the keyboards to DVORAK. Of course, people are scared to death of popup messages. Play around with the shutdown command.

If you wanted to do something way less problematic, buy some of the gadgets on thinkgeek.com or elsewhere; I've seen one that adds extra keystrokes randomly (plug between keyboard and box) and one that emits this annoying beep randomly that you hide under the desk. ANNOYING.
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