Let's see.....a few I have done are:
1. Tape the mouse-ball so that it will not roll (or just remove it all together). This usually leads to them rebooting a couple of times trying to get it to work again.
2. Tape the button(s) under the hand-piece of the phone down. Use clear tape so that it cannot be eaily seen. When they go to answer the phone, it will just ring and ring and ring! OR
3. Unplug the phone cord just enough that it looks plugged in but when they go to answer the phone, the cord will come flying out of the phone. This one, when used in conjunction with #2, usually works well.
4. Depending on the type of filing cabnets that are available, I have rigged the file drawers with large rubber bands so that when a person tries to open it, they only get it so far before it shoots out of thier hand and slams back shut.
5. For those who have cubicals with overhead storage, I once rigged a bag full of packing peanuts to turn up-side-down, when the door was opened, so that they poored all over the victims desk.
6. I work for a major business forms company so I have pretty much unlimited access to the little round pieces of paper that are left over after a form is punched. These can lead to endless possibilities of confettii carnage.
It's the simple, yet effective, ones that are the most satifying.
I hope these help a little.