Get a few bottles of Pepsi and Coke. Each of your guest gets to be a tester by turn. Give your tester 3 glasses. Fill two of the glasses with, let's say, Coke and one with Pepsi. The tester is *not* required to say which is Pepsi and which is Coke. All you they need to say is which one is different from the other two.
In the beverage business, this is called a triangle test. ... Believe it or not, you will find this task incredibly hard. If a thousand people were to try this test, just over one-third would guess right - which is not much better than chance; we might as well guess.
Of course, Gladwell does not recommend this as a party game. I just thought this could be one.
If conducted with flair it could engage your guests for some time. The success lies in everyone having equal opportunity to win.