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!
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. I might actually make this! You can loop it over a tree branch and let kids bat it around for more watery fun!
Instead of water balloons make sponge bombs! This is a great way to beat the heat without using up a lot of water, and sponge bombs can be used and re-used all Summer long. (And don't have all those balloon pieces everywhere!
Outdoor tic tac toe using stone tiles and rocks. I would make a small garden for the kids and have this in it. Cute decor when the kids aren't playing with it. Great for when the mom is at work and the kids want to play