A small city with a great number of Mid Century buildings is Columbus Indiana. Our family went on a tour there a couple of weeks ago. Buildings designed by both Eliel and Eero Saarinen, I.M. Pei, Harry Weese and others are among six modernist buildings designated as National Historic Landmarks. It’s a town of about 45k and has around 70 building considered architecturally significant. All in all a nice weekend trip if your in the area.