There are tons of great parks in Iowa. I worked with the iDNR as a summer intern. I traveled the state looking at alot of the state parks and facilities doing some environmental analysis. Redrocks near Des Moines and Mines of Spain in Dubuque, Ia. Are great!
If I can I would like to make a suggestion, geocaching is a great way to discover places one person may not know exists. All you need is a GPS unit and the website
Geocaching - The Official Global GPS Cache Hunt Site Try it out its one of my favorite pass-times.