My favourite one is the hollow-face illusion, but I won't give it away by describing it. A related activity that demonstrates this effect is this dragon's head optical illusion [via Mindhacks] which I've made myself. For some people it takes a while to see (some people just peer and it and say, "so what's supposed to happen?"), but eventually most people seem to get it.
Mindhacks, by the way, is the blog for the book Mind Hacks, which I've just bought and can't wait to get into. It looks pretty good.