I am not sure if this is the right forum. I am sort of guessing.
I have been reading a lot around RTA lately and maybe I have just been unlucky, but the only thing I see mentioned regarding St. Louis is the Arch and the trip up it. I do have to say that I have lived here for 13 years now and have never actually gone up in the Arch. That being said, here would be my suggestions for the St. Louis area beyond the Arch:
The Cahokia mounds
Ted Drewes Frozen custard (the one on old 66)
An Italian-American meal on The Hill. Many choices here. My personal favorite is Zia's, but that is largely because it is across an intersection fromthe Missouri Baking company and some excellent canolli
The Budweiser brewery tour
If you have kids, there's the zoo (free admission and, if you're a cheapskate like me, free parking either on the streets nearby or at the Art museum), the Magic House, and Grant's Farm
There is a hiking/biking path across the Mississippi River just North of town at the Old Chain of Rocks Bridge.
Everyone talks about the old river town of Hannibal a ways north of St. Louis. I see little talk around of either Ste. Genevieve to the South or of the winery region (Hermann, Washington, etc.) West along the Missouri.
Also, if you are looking for homestyle meals which make no pretense at being low-calorie along with some very good pies in a tourist-y (some would say kitsch-y) small old river town, there's the Blue Owl Restaurant and Bakery in the town of Kimmswick just South of St. Louis.
And this all is just really just scratching the surface.
All the best,