Another good one...
Military folks all have these fancy new id cards that we use to log on to computers and stuff. Well, we arent supposed to leave them laying around so when my guys do, it gets put into a cup of water and then in the freezer. (or outside if its winter). They end up with a nice id card block of ice in about an hour.
Welding their lockers shut at the shop is an instant hit at the end of the day too.
Stapling or sewing a sleeve closed on coveralls is always fun. Just have to make sure you know which arm they always put in first...leave that one open, that way they dance for a minute trying to get it back off.