If it's a wireless mouse, take out the batteries, and tape the laser
light below it (or take out the ball if it's an older-style mouse).
You can do the same with keyboard, either unplug it or take the batteries out if wireless.
Change their desktop background photo to something funny..
I've heard of someone having taken a photo of their screen with all documents and interenet pages open, then set it as background photo and closed all the programs.
Change their Internet home page to something funny.. like and Adult site.
Be carefull 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.
Go into their "Word" program and change the "Auto-Correct" settings to some funny words, so that when they type the company name, it automatically changes the name to a competitor's name.
Add extra letters or reverse the spelling on many commonly used words.
And finally, make some of the most used words to change to "April Fool"!
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"
Make a few copies of paperclips 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.
If you have kitchen at work, swap the lids of salt and pepper.
If the kitchen sink has a spray nozzle, use a rubber band to secure it in the "on" position and then ask someone to get you a drink of water.
When the water is turned on, they get sprayed!
Go to work a little earlier in the morning.
Set you office clocks one hour early and when people arrive, tell them they are late.
-- April Fools Office Jokes --
Pick up someone's car keys and go and park their car at a different spot, then put the keys back.
Before your co-worker arrives at the office remove his desk drawers and switch them around.
He will definantly scratch his head over this one.
Hint: In case you cannot remove the drawers, just remove the items and swap them.
Hide their phone and then call them.
Record the reactions with a hidden camera, then send them in to Funniest Home Videos, you may win a prize!