Hide a "cap" (mini-firecracker made for toy cap guns) under the handle of your victim's stapler.
Print out some pictures of a scantily-dressed girl, or other racy image. Mix them into the paper tray of the office copier or printer. When the victim makes copies, they'll have a big surprise!
A Call From Above
If you work in an office with a suspended ceiling: Hide his phone in the ceiling tiles above your victim's desk. When they arrive...give them a call!
Submitted by Drake
Have a Seat
Take all the chairs you can find from other offices, the conference room, etc., and fill up your victim's office as full as possible.
Oh So Pretty
Replace everything on your (male) victim's desk with similar items, but make them as "girly" as possible (pink pens, lacy picture frames, bows, flowers, etc.). Remember to change his screen saver too!
Submitted by Joe S.
Tied in Knots
Take one or more balls of string or yarn. Wind them all around the victim's desk, chair and furniture, connecting everything together. They'll have to cut open a path!
Pleased to Meet You
Do you have access to your co-worker's business cards? Have some fake business cards printed up, with a wacky job title, funny photo, etc. Plant them in place of his/her regular cards and wait until they try to hand one out!
Move someone's desk, with all belongings, computer, chair, phone, etc., intact, into the rest