Today is April Fools' Day - finally, the one day that it's completely acceptable to do awful things to the people you care about and get away with it because of some bizarre medieval tradition.
1. The toothpaste treat trick

Buy Oreos for the office. Then laugh at their bewildered and disappointed faces as they realise you've filled their biccies with toothpaste. You MONSTER.
2.Don't hide in the toilet, but pretend to instead

3. Make them think their screen is cracked

Find and download a reasonably high-resolution image of a cracked screen. Searching on Google images should give you lots of options, or you could just use the one here.
Set the image as a screensaver on their computer, making sure it is set to activate after a short amount of time (30 seconds or so).
If you colleague uses a desktop computer rather than a laptop, you could make the prank more convincing by unplugging their keyboard and mouse.
4. Light crawler
You can order these online or simply make your own (out of something non-flammable, naturally).

5. (Temporarily) render their mouse unworkable

This one is deviously simple.
Just stick a little bit of tape over the laser on the bottom of the mouse and it will stop working. Most people won't think to turn it upside down and look for a good long while.[Mirror]