Depending on transportation and such you might want to look into the Science Center. It has wonderful hands on science exhibits geared toward kids. I'm sure it's nothing you don't know, but it might be a fresh way of presenting some things. It also has a wonderful IMAX movie theater.
St. Louis Zoo is also one of the top 5 zoos in that nation according to whoever decides these things. I can't really compare because it is the only zoo I've ever been to, but it is very nice.
The list of fun things to do in St. Louis could go on for pages. It is a big, active city. I'm certain you will be able to more than fill up any free time you may have.