Kalahari: Just visited this week for the first time. This place is awesome. My kids are both 6 and they had a blast. At this age they can do just about everything there. Best indoor water park in Ohio. Great way to beat the winter blues!
I love the lazy river at Typhoon Lagoon. When you get too tired being thrown around in the wave pool, you can just float along on a tube. It's also nice because you can just hop off your tube and get pulled along in the water.