Roll up! Roll up! If you're a kid at heart, head for Iowa State Fairgrounds. See if a fair or event is being held during your stay.
If you love hearing the word "free" as much as most people do, you'll be glad to know this outdoor exhibition area is precisely that. Iowa State Fairgrounds is located in Des Moines. If you want to keep your travel schedule full while you're around these parts, also stop by Adventureland, Greater Des Moines Botanical Garden and Iowa State Capitol Building.