I agree with everything said above. The museum below the arch is very informative about the beginnings of the development of the west. Right by the Arch is the "old" Cathedral. There are two cathedrals in St. Louis, the old and the new. The old Cathedral is the first church west of the Mississippi. It's a beautiful church. If you have the opportunity, go to The Hill. There is a restaurant downtown named Kemolls. It's an upscale Italian restaurant that has been in the city for years. Excellent food!!
Enjoy yourself in my hometown!