Balloon Rockets - I've done this in several science camps I've conducted. Kids have fun with it. But, when the balloon deflates it may fall off the straw because it's surface area changes - another good thing to show and explain to kids.
"The most powerful tool in coaching is the question - the Greek philosopher Socrates is regarded by some to have been the first coach. Among other things, he used deversified questions in his talks with his students. Here is how he did it.