This EPIC Bottle Rocket Flew Higher Than our Two Story House! Use a soda bottle, baking soda, and vinegar. Super fun science and backyard project! Demonstrate Newton's law of motion. Also put so glitter in it for a fun explosion
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.