When the day gets too hot and the kids a little restless, I just put out a couple of buckets half-filled with water, rustle up some toys for scooping, and away they go! They love to use the little watering can to create rain, and scoop water from one container to the other. It's great motor skills practice for younger children too. This kept my girls occupied for an hour! Just make sure they're watched all the time.
Note: NEVER leave kids alone with water. NEVER leave kids alone, period.