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.
Fourth of July Firework Salt Painting - Busy Mommy Media _ This is such a cool art project for kids! It looks so neat when it is done. I love that it uses stuff I already have on hand. This would be awesome for a rainy day activity!