The Gateway Arch of St. Louis is probably the best...

The Gateway Arch of St. Louis is probably the best infrastructure of Missouri. They have construction going on right now so I strongly advise to buy ticket online. They always have huge line there at ticket counter.
We enjoyed our trip. They have small museum at the basement of tower. The trip to the top of tower was awesome. You can see stadium and most of St. Louis downtown from the top. The lighting in St. Louis Downtown looks amazing at night so buy ticket around 7 PM.
Parking might be very tricky so try to find a good location to park before you go to the arch. There are some parking at riverside but they close after 5 PM.
At river, they have helicopter ride and cruise. Both of them are amazing. If you plan to ride those as well, get your ticket online.
Enjoy your trip.