The Ketchup Experiment! Kid friendly site with lots of neat experiments! It has one that makes a rocket out of a film canister, which would be good for an grade space unit, but it has to many kid friendly expiriments. Great site for any science teacher.
Blow up a balloon with yeast or baking soda and vinegar. Fun science project to do at home! Put vinegar and baking soda in a bottle, put the balloon on the opening, and watch the gas from the chemical reaction blow up the balloon!