Cool the kids off with a homemade sprinkler. Take a 2 liter soda bottle, poke holes in it. Attach to a garden hose via a male to male adapter that cost less than $5 at Lowes. Toss over a tree branch and let hang. You can adjust how the water sprinkles by adjusting the water flow. When the bottle gets stupid, just switch it out.
Camp Mom! 20 Activities to Make Summer Awesome for Everyone
Scatter balloons around the yard and use swimming noodles cut in two to move as many balloons in the laundry basket as you can in the time allowed. Only the poole noodle can be used and kids will soon realize they have to cooperate to get the balloons in the baskets.