Take a 2 liter soda bottle, poke holes in it. Attach to a garden hose. Toss over a tree branch and let hang for a kids water sprinkler. You can loop it over a tree branch and let kids bat it around for more watery fun!
Here is a sure fire way to WOW this Summer! Make your own glow in the dark chalk using only two ingredients plus water. This chalk glows in the dark, under UV light, and it even works in broad daylight! (SUPER Summer FUN from Growing a Jeweled Rose)