When you’re in South Dakota, make sure you visit the Badlands. There is a campsite or two that is away from the main part of the park that is awesome to spend a day or two in - if only to interact with the wild roaming herds of Bison. And Boot Hill in Deadwood, especially Wild Bill and Calamity Jane’s graves (for the historical significance).
The badlands in ND not to be missed, Teddy Roosevelt National Park, the Marquis De Mores home, Medora, ND. These badlands are much more beautiful than than the badlands in SD.