Lost Island Theme Park is a large amusement park in the Cedar Valley, organised around five Polynesian-themed realms. The park has roller coasters, a log flume, family rides, and a dark ride, making it one of the area's strongest stops for travellers with children.
It is outside central Cedar Falls, so plan it as a half-day outing rather than something to tack onto a downtown walk. Check the operating calendar before going, since Midwestern theme parks are strongly seasonal.