If you have kidlets, The City Museum in St Louis is terrific! As is the Magic House.
The Annhueser-Busch brewery tour, Grant's Farm, the St Louis Zoo and the Science Center are all free. The Science Center does have certain things that require an additional fee (like the planetarium). The Arch is nifty if you've never been there before, and the Cahokia Indian Mounds are really something.