Fontana Park in Hazleton, IA has tons of things to do. From live animals including buffalo, eagles, and a bear to name just a few to fishing, hiking, biking, camping, and even eco friendly cabins. Pretty cool place in the middle of nowhere. My kids also loved the real beehive in the interpretive center. Other than the overnight cabins and camping, everything is free.
https://www.mycountyparks.com/County/Buchanan/Park/Fontana-Park.aspx
If you wanted to do an overnight, you could also hit Backbone State Park in Strawberry Point which is very close by. That one I believe has an entrance fee since it is a state park. Some of the coolest trails and caves in the state.