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.
Outdoor Summer Fun and so did we. we saw this list! Some new and amazing ideas we haven't seen yet to keep kids busy this summer. Worth a look for the links alone! 30 out of the box ideas (and awesome links to support you in doing them.
32 Fun DIY Backyard Games To Play (for kids & adults!)