32 Fun DIY Backyard Games To Play (for kids & adults!) this has some of the best outdoor ideas I've ever seen. Will be trying ladder bean bag toss and sidewalk/box board game
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.