The St. Louis region has Soldier's Memorial, Jefferson Barracks and the new Civil War Museum, the Missouri History Museum, the Art Museum, an outstanding antique airplane museum at Creve Couer airport, Cahokia Mounds, The National Transportation Museum, Laumeier Sculpture Park, Grant's Farm and his home, the site of Fort Bell Fontaine, the KATY Trail, the World Bird Sanctuary, and the Wolf santuary next door, Melvin Price Locks and Dam at Alton, the confluence of the Mississippi and Missouri Rivers at Confluence Point State Park, the Audubon Center at Riverlands Migratory Bird Sanctuary (and now is a great time to go), and no doubt something else obvious I have overlooked.
There is an important Civil War site in Alton, Illinois; I'll leave that for you to research.