This is really funny to do in a restaurant restaurant must have plastic menus).

Order a glass of water, and put the menu on top of the cup. Holding the menu against the cup, turn the cup upside-down, so that the cup upside down on top of the menu. CAREFULLY pull the menu out from under the cup and and stick around to see the expression on the waiter's face. (they'll have a really hard time getting the cup off the table without it spilling!!)

Put some flour on the blades of a celing fan - wait for someone to switch it on.

Lemonade with hot chilli powder makes for a very realistic looking ginger beer...