St. Louis is a great city in the Midwest. The zoo is one of the top-ranked zoos in the nation, and it has free admission. The art museum and science museum are great--and free! I think it's like a mini-Chicago where all of the great museums are free.
Our favorite thing in St. Louis is the City Museum, which is not a museum at all. Your kids would love it! It's a crazy indoor play area designed by super creative artists. It's like nothing we've ever seen.
Check it out here.You could head south from there and see a scenic park of the state with a visit to Johnson's Shut Ins and Elephant Rocks, both of which have really unique natural rock formations.