If you know someone that uses an optical mouse, place a small post it note over the eye of the mouse. When your victim tries to use their mouse it won't work. When they decide to look underneath it, they will see a small posted that reads, "April Fools"

Ypto Ypto...errr!!! TYPO!
Be careful with this one, as to not ruin the keyboard. All you have to do is simply and very carefully remove a few keys and switch them around. If your victim is one of those "look up and down" typers. He or she will be very confused while trying to get work done.

Where are those paper clips?
Make a few copies of paper clips then put those copies back inside the paper tray. Whoever uses the copier next will get their copies with those paperclips on them and will look all over trying to hunt them down.

Phone Label Swap
Change the speed dial labels on office phones. It will piss off your co-worker when he is trying to call tech support but instead speed dials the courier.

Self Recognition Prank
Without disguising your voice, compliment yourself over the office intercom. Compliment yourself on a just completed task or give yourself the rest of the day off.

Stale Donuts
About a week before the prank, buy a box of donuts. Keep them in the refrigerator with the top open so they can dry out. On the day of the prank, place the box of donuts near the office coffee pot so anyone with a morning sweet tooth can indulge.

Pick Up The Entire Phone
Wrap a very small piece of wire around the phone cord so both ends are tied together. When the phone rings and your victim goes to pick up the receiver, he will up with the entire phone base with it.

Confusing Drawers
Before your co-worker arrives at the office remove his desk drawers and switch them around. He will definitely scratch his head over this one. Hint: In case you cannot remove the drawers, just remove the items and swap them

Pen Swap
Take all of your co-workers pens and replace them with your pens that your glued caps on to the night before. He will not be able to take the caps off to use the pens.

Old Toy
Use an old toy that has a button which is used to activate sound. Such as a baby crying. Then tape it to the back of the victims desk drawer, but don't close it completely, leave it open just a crack. When your victim decides to close it, he will hear the annoying sound and wonder where its coming from.

Something is wrong with the copier!
Leave a copier to print 99 copies at 33% resolution on 8x14 paper.

Upside Down Cup
Fill a cup with water about half way. Then place an index card over the cup. Then turn the cup upside down on your co-worker's desk. Finally, carefully slide the index card out from underneath the cup. Whenever your co-worker decides to pick the cup up, he will be drenched in water.

Auto Correct
Microsoft Word has a feature called 'Auto Correct'. It will auto correct any mispelling such as 'TEH' which will automatically be changed to 'THE'. Anyone can go into the tool and change the words. When your co-worker is away from his desk. Change a few of his common words around.

Hey! I wanted snacks!
Empty boxes of common snacks such as Cheez-Its or Figs and replace them with shredded paper or anything else that will weigh the box down. Then place them in the break room.