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.
32 Fun DIY Backyard Games To Play (for kids & adults!)
Balloon ninjas to shoot with nerf guns. This is a fantastic idea! Great for little boys! I know this was posted as an idea for little boys. but I think I'd like to have a nerf gun battle with these ninjas.