Take out all the stuff from the boxes, wrap them nicely, and stack them at your co-worker's desk so it looks like they have a bunch of presents. Then, when they return all fresh and happy from vaca, they see a bunch of presents!!! But then, they open them, and BOOM. Nothing but emptiness and broken dreams. At that point feel free to walk by with the last of the coffee and snarkily welcome them back to work.
Pfft. Child's play. Previous pranks:
-- We filled his cube with assorted fake greenery (I think they were Christmas decorations), stuffed animals (our company logo is a bear, so we have a *lot* of stuffed bears of various sizes all over the place), and a really creepy Oregon State University (our boss's alma mater) garden gnome. I was the instigator of this one. I have the photo around here somewhere.
-- Someone left a printed-out email taped to his computer monitor that said that he had won an iPad in a company-wide giveaway. That one was fun and accidentally hilarious because he *insisted* it was a joke being played on him by a certain coworker. I swore up, down, and sideways that it was *not* a joke being played by that coworker. Because it wasn't. It was *our boss*. He refused to believe that she had done it.
-- A coworker sent out email late on the Friday before his return saying that it had been nice working with everyone, but it was her last day, so she was going to miss everyone. That one was a mean success because she was coming in late on that particular Monday morning, so he almost had a breakdown coming to grips with the fact that she wasn't going to be there any more, but he buckled down and tried to figure out how to cope without his office mom around. He just lit up when she walked in the door and told him that it was a joke.
-- The one I'm proudest of: This guy *loves* to snack at his desk. An email went out "reminding" everyone to make sure they took some new office training (I can't remember what we called it) so they had certification to eat snacks at their desk. The stroke of genius: If you didn't have your certificate posted on your cubicle wall at your desk, you couldn't eat at your desk any more. All of us but him had our certificates because, well, we were all in on it. His first day back (a Monday), he decided he got a day for a grace period because he was coming back after being gone and had to catch up on stuff. On Tuesday, he specifically said, "I'm going to break the rules and eat at my desk today! I'll do the training tomorrow." (Our Mondays and Tuesdays are hellacious, so it was actually reasonable for him to delay anything other than our core job function on those days.) On Wednesday, he had caught up on his work enough to go hunting for the training. And he hunted. And hunted. And asked several different people, all of whom said variations of "I don't remember where it was. It was just there when I logged in. Can't you find it?" Which is in fact what most of us say whenever someone mentions *any* training because, well, we *don't* remember anything about the training other than the fact that we finally took it because the deadline was *that day*. Anyway. Finally, he was taking it so seriously that someone broke down *after three days* and told him that it was his welcome-back-from-vacation joke. I think I still have my certificate stashed away in my desk somewhere.
There have been a few other smaller pranks, but these are the big ones that come to mind. Seriously, the certificate thing *still* makes me smile, and it's been a few years.