Do you plan to visit the same area, or check out a different part of the US?

If you visit the Midwest, I might recommend visiting the Boundary Waters area in northern Minnesota, checking out the sights in the Duluth region, and then a day trip to see Sibley State Park near the center of the state. I didn't make it to the Boundary Waters this summer, but I wrote about the rest of my suggestions.

Sibley State Park
Various hiking trails and walking paths across Minnesota
The Lake Superior Railroad Museum
More travels in the Two Harbors and Duluth Area