The best: Swapping packing peanuts for Cheetos in a dimly lit room.
The coffeemaker was in the machine room where I worked, and the lights were never on, because people needed the room dark to see the projectors they worked on. So one time, I got an empty Cheetos bag and some Styrofoam packing peanuts shaped just like Cheetos. I dipped them in water, and then in the cheese powder from a box of mac-n-cheese dinner. I put them in the bag and left it open beside the coffeepot with a sign on it that said, "Do not eat, property of so-and-so."
I hid at the back corner machine and just kept quiet when anyone came in. I heard some interesting noises when people tried the fake Cheetos. Everyone agreed it was a good prank.
I'm guessing Monster posts these hoping some folks will try similar stunts and then be looking for new jobs April 2.