My kids love to water the driveway and watch the water evaporate, but I hate to waste the water. Our solution? Ice painting! We get a large bucket of ice, and draw with the cubes like chalk. They love writing with ice, and watching the letters and pictures quickly disappear.
Another idea: fill a bucket with water and let the kids "paint the house" with a clean paintbrush or sponge.
What's your favorite summertime water-play activity that doesn't involve thousands of gallons down the drain?