If you can manage the hike to the top of Black Elk Peak (formerly known as Harney Peak) it is worth the hike. It is beautiful. Drive the Iron Mountain Road in the Black Hills and drive the Wildlife Loop in Custer SP. The Badlands are worth driving through, stop and look, don't just drive through. The Notch Trail is worth the hike if you don't mind a very rustic ladder you have to climb.
When we drive from WI through SD, we avoid I-90 and prefer to take the US highways. The people in the small towns love tourist spending money so they will talk with you and make you feel at home.
