Spray bottle makes cool (and water-saving) fun during the summer
Annmarie's hack uses far less water than the kiddie pool or the sprinkler:
Buy a 24oz empty spray bottle and fill it with water. [Check the bottle to make sure the trigger is easy for small hands to pump. -- Ed.] Let them spray all the plants, bushes, windows, matchbox cars, etc... outside. My 2 year-old will spray for hours on a hot day. And, it takes forever to get through the entire bottle of water!
Spray bottles make great portable air conditioning as well. Quick way to cool the kids off on a hike or a hot car ride. For longer cooling, get the kid's hat good and damp (that was always our trick on long hikes when I was a kid).