There's a very funny idea on my mind which I kept for the April Fools Day. I want to play trick on my co-workers.
That day I will be anxious because of a fictitious report I will have to give to the boss. But indeed I will go out to get prepared for the prank. I'm gonna get some graphite, mash it up and apply it under one of my eyes. I will do my best to make the "bruise" well visible. I'll spend some time out of the office trying not to get caught.

Then I'll make a mess with my clothes and come back to the office. I will dolefully enter and say that I have been beaten because I haven't finished my report on time. I can imagine their anxiety and haste with which they will be finishing up their own reports :)