vinegar and baking soda in a bottle- blowing up a balloon
and, of course, the mentos/diet coke geyser.
We had a great turnout, and all the experiments went really well. Alex dressed up as a mad scientist to answer the door and had "Weird Science" blasting through the speakers. I was too busy to get any pictures of the action (which is too bad, especially for the mentos experiment), but here is the aftermath on the patio table.
Messy, but very fun.