The top answer is pretty good:

I filled the window of my boss's office with styrofoam peanuts. From the outside, it appeared the whole office was full. From the inside, you can see they were just held in place by bags. I set up a camera in the hall to record his horrified reaction as he walked up. The best part was, minimal cleanup.

Picture:

Overall the Reddit ones are better than "Parade picks":

http://www.parade.com/parade-picks/2013/03/31-april-fools-day-tricks-to-try-at-home-and-work.html

Their best suggestion is cracked iPhone background:

I don't vouch for any of these:

https://pinterest.com/kashaw/april-fools-pranks/

Or these:

All of Google's April Fools jokes:

http://thenextweb.com/google/2012/04/01/googles-april-fools-jokes-continue-meet-really-advanced-search-and-multiple-cursors-for-chrome/