A few more I remembered - I'm sure I saw these on the internet a while back:
Buy a box of donuts on Friday Afternoon - leave them out over the weekend, then put them out on Monday with a "Free Donuts" sign on the box.
Take apart all the pens and remove the ink. Or, put red ink sticks in he black pens, etc...
Put a fake plastic fish that looks dead in the water cooler.
Dump the paper holes punched from a hole-puncher into an umbrella while it's closed.
Remove mouse balls.
Saran wrap the toilet.
