If I had a MacBook, I would definitely do that "Falling Apart Desktop" prank.
I also like the "Desktop as Wallpaper" prank as well. I've done that a lot in the past.
One time, I was working at NSC at NTID. I was Director of Communications. (This position involves computer work.) The only other person who was familiar with this prank was the Webmaster (also a friend of mine). What I would do is go to all the computers and take a screenshot of whatever was showing. Then, I would use that picture as the wallpaper... hide all the icons... hide the bottom bar... then leave. Later during the day, i will see people lined up to use the only Mac in the office (there were 5 PCs and 1 Mac) cuz none of the PCs were working. Of course, my friend and I would be laughing. Since we both knew the shortcuts for everything, we could switch around quickly without anyone noticing. Hehehe!