The best kid toy for the summer is hands-down this: a squirt bottle.
No, not a squirt gun. A bottle, like a salon uses to wet your hair. My neighbor got one for her two-year-old, and my kids seemed to like it. After a trip to the Dollar Tree yesterday, we had two.
Those things kept Edith and Atticus entertained for at least an hour, watering plants and watering themselves. And the spray is so fine that when they inevitably watered me too, it wasn't even that bad.
Another bonus -- it holds a lot of water, so you're not refilling every two minutes. Nevertheless, their bottles were refilled at least twice. The squirt bottles are already a summer staple.