I've seen/been involved in many office pranks over the years.
One time, one of our co-workers was gone for a week on his honeymoon, so we decided to fill his cubicle with a thousand dixie cups half filled with water. Unfortunately, we did it the night before he got back, and by morning, most of the cups had started to leak. Our facilities team was not amused.
Another time, one of the managers was gone for a few days, and we filled his office with 9 industrial size bags of packing peanuts. Office was about 10x10 or so, and the peanuts were filled about 3ft up on the floor.
And the best one of all,