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.
How to keep a child busy for hours in the summer: DIY: Giant Ice Cube Awesomeness - filled with plastic animals, dinos, and such. And I'm sure you could put in food dye before its frozen to color it like the ice block in the photo.
Make a backyard bowling alley. Add food coloring to clear plastic bottles of water. Use party streamers as lane margins. Add glow sticks for night bowling. (These would make great outdoor table nightlights also with the glowsticks, gonna try it!
Need to remember this - a balloon curtain for kids to wake up to on their birthdays! Another take, once all guest arrive, hang balloons and the guest of honor walks through the balloon curtain. or any party/bridal shower