I went around the office and emptied all the 3 hole punches into a large cup. I then took all those little round paper dots and put them on a piece of paper inside a co-workers overhead cabinet, and taped the paper to the inside of their overhead cabinet door.
Later, when he had sat his cup of coffee down with the lid off, I asked him to borrow a manual that I knew was in that cabinet. POOF, snowing. All in his coffee, his keyboard… good times.
We also had this rather ditzy office manager (aka, secretary who was given a fancy job title because she had been there so long). I wrote a little VB app that would play random ghost like sounds at a really low volume at random times. She actually got up and ran screaming from her office.