Scott's Birthday balloons reminded me of a couple cubicle pranks I have been indirect party to in the past, and I am sure there are other good ones other people have...
Replace the opening to a cubicle with a solid panel. The victim actually liked it sort of. He found a milk crate to climb in and just stood on the desk to jump out. Got a ton of work done with no "drive by" annoyances.
Cover opening to a cubicle with plastic & packing tape and then fill cubicle with packing peanuts. This was funny, but did not go as well for the victim or culprits. Clean up was a bitch.
We played with moving the flat surfaces in a cubicle up or down a couple inches or so, but all the crap piled on them made it way to labor intensive for practical use.