Last summer, the kids visited Full Blast water park in Battle Creek, Michigan: http://www.fullblast.org/
There was an indoor park and an outdoor park. There were two slides inside and two slides outside. The indoor park had a whirlpool area and a lazy river.
They had fun throwing water balloons and a bungee cord area to run around and be pulled back. Some kids enjoyed bungee jumping.
Rating: 7/10 because the water outside was cold and the weather was cool.