Here are some awesome and ingenuous Office Pranks. Enjoy!
Wrap the office in aluminum foil
Replace it with cardboard:
Care for a drink?
"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)
Paper yields itself the best for these kind of projects -
Innocent paper gives way to less innocent games:
Spice up the company photo
Office Washroom Pranks