Hammer hit head protected by "paper" or metal plate
(This demo is not hurtful, but can produce a loud noise.)
Take the rubber mallet and a sheet of paper. Ask a student to take the mallet and hit you on the head while you protect yourself with the sheet of paper. (ACTUALLY, STOP THEM AFTER THEY SHOW SURPRISE AND BEFORE THEY HIT YOU!) It would, obviously be harmful. Next have them hit you on the head with the heavy aluminum plate.