The Ten Cent Game
At the beginning of the celebration or whatever all partygoers must be notified of the rules.
Whoever starts the game will produce a 10 cent coin and at some point (preferably quite early) he/she will drop the 10 cent coin into someone's drink and that person must down that drink.
That person then must drop the 10 cent piece into someone else's drink and they must down their drink and so on.
As the game goes on people get more drunk and forget about the game and forget to guard their drink,
additionally people lose track of who is in possession of the 10 cent piece and will not expect it to be dropped into their drink.
WARNING: be careful not to swallow the 10 cents, this may cause damage to one's insides...
but it is very amusing when someone does.