Penny dropping is a good one.
You bet the victim that he /she cannot drop the coin that is resting on the victims forehead (head should be tilted back for this) into a funnel that is wedged down the front of the victims trousers/skirt etc.
Basically all they have to do is bring there head forward, the coin slips off, and falls into the funnel, quite easy really.
They have to do this 3 times to win.
Now the fun bit, after a couple of attempts, and with victory in sight, the victim will fail to notice the glass of water that you are holding, so with head back all that remains is for you to empty the contents of the glass into said funnel, then stand back and watch the ensuing drama