The place I used to work had a little, plastic bicycle horn; the kind with the rubber bulb on the end that you squeeze to honk the horn. It was used as a signal. If you needed everybody’s attention for something, or just for our morning meeting, you’d honk the horn. One night I was the last to leave the office, so I filled it up with water and put it back on the hook.
Get the dorkiest picture possible of your boss. Stay at the office late one night and set the background wallpaper on everybody’s computer to that picture.
Find a cow-orker who uses their CD-ROM drive a lot. Get a spare CD-ROM drive and put some obscure CD in it. When nobody’s around, open that person’s computer and mount that new drive inside it, but use one of the internal hard-drive bays so it won’t be seen from the outside. Hook the data cable to the hidden drive, but be sure to leave power connected to the original CD-ROM, too. Close the case. The regular drive will still work since it has power; it will open and close. But no matter what CD someone puts in it, the computer will read the hidden CD inside.