We are in Duluth, and we've made this trip a couple of times, most recently just last September. For variation, we went out through North Dakota and returned through South Dakota.
Other folks have already listed some great places. Corn Palace, Badlands, Wall Drugs. While at that exit, go a few blocks further west and visit the National Grasslands visitor center. Very nice exhibits on the grasslands, and very helpful staff that can direct you to some neat places. Minuteman missile site - they will have their new visitor center open in 2015 and it is right on the highway. Theodore Roosevelt NP was one of our favorite stops. See both north and south units. Dinosaur museum in N. Dakota. Don't miss the town of Medora, ND. Great historic places, a wonderful stage show and steak cook out.
If you're going to the Black Hills, try to do a meal at the Circle B Chuckwagon. Great food and wonderful family entertainment (family plays and sings western music). Bear Country USA, Wind Cave NP, Custer State Park. There are several scenic byways in both North and South Dakota that will take you off the interstates and let you see some of real America. You can find these on the internet, or by visiting the websites of the N or S Dakota tourism commissions, which I highly recommend that you do. They will send you loads of info.
