-- Shocking World --

"Take a spare, dead keyboard that's identical to the victim's, remove the keys, fill the keyboard with dirt, and plant "cat" grass (from pet store), which is thick and grows fast. Then cover the keyboard in plastic and keep it in a sunlight. After 18 days, the grass had grown several inches and even uprooted a few keys." (Techrepublic) Another one:

Replace it with cardboard:

Plan Your Sweet Revenge! Here are some awesome and ingeneous Office Pranks. Enjoy! Wrap the office in aluminum foil