Add sand to keyboards.
Keyboards have a natural grate-like nature, which allows food and other debris to fall between the keys. For an illustration of this, look between the keyboard of someone who routinely eats at their desk. There is a high probability that you will see crumbs that have collected underneath the keys. This will be the tray that collects the sand that is added.
Choose between adding a small amount of sand each day, or completely filling the keyboard. In either case, be sure to wipe away any residue that may be on the surface of the keys before leaving the area. For best results keep a canister of alcohol wipes in your desk. They clean well and dry quickly. These are usually available in any stock room – they are highly coveted by the germaphobes and whiteboard destroyers alike.
Give your prank a fancier feel by replacing the sand with glitter. Glitter is a great option for keyboards that may be near a window, air conditioner or open door. With a bit of luck a dull Thursday afternoon becomes a glitter ball with a slight breeze.