One of the coolest audience participation activities I've ever seen was a person who - without speaking a word - had the audience divide into quarters and each make percussion sounds (hand clapping, finger clicking, foot stomping and hand rubbing) that sounded together like a rain storm. It was awesome, and I'd love to try it again.
Mokalus of Borg
PS - Unfortunately, I think it requires a few more people than I will ever be leading.
PPS - The original event took place with about 300 people.