Oooh, here's a good game invented by David Cross. At the end of a Happy Birthday song you go, "..toooo youooooo" and then "Haaa-a-a-a-a-py Birthday to you.." and sing it again. Then, when that one's done you do it again. Then you do it again only this time some other people are coming in at the wrong time like Frere Jaques (but less harmonious) and so on and so on until people start getting pissed. One time we did it until the manager of the restaurant kicked us out going, "All right guys THERE'S A FUCKING LIMIT!" Priceless.