Here is a couple that are safe and sound...
Get a urine specimen cup (if you are worried about taking a 3cent coup from the hospital, ask them theyll laugh and give it to you) and fill it with apple juice. Warm it up in the microwave and sneak it into your Supervisors drawer or drop it on his desk... good laughs.
Or get an adult diaper and two bars of snickers (or similar); nuke the bars in the microwave and stir them up nicely, while still hot spread it on the inside of the diaper -> thus resulting in a nice code brown. Put that in a file cabinet or drawer with 1-2 inches of the diaper sticking out, thanks to human curiosity someone will pop that drawer open...
We pulled both pranks on our Sups and Managers, the general managers veins started popping out of his forehead, it was priceless.