as in first posters reply there is a lot to do in St. Louis, but if your days are not tied completely to St Louis ( and you have a day to spare) you might try going a little farther north and go to Hannibal, MO!!! yup home of Mark Twain, see fun info about Tom Sawyer-Becky Thatcher and Huck Finn,
there is a riverboat called the Mark Twain serves a delicious dinner while cruising up and then down the mighty Mississippi.
See the Unsinkable Molly Browns birthplace
whatever you decide to do have a wonderful and safe trip