We stopped at the museum/information center about the the ICBM sites that use to be in the Dakotas.  Don't know the ages of kids... the film might be too traumatizing for them talking about how close we came to Nuclear War.    It will only take a couple of hours at most unless you want to tour a ICBM site.    Explains all those  bee hives behind the fences as you drive out there.  

We stayed at the Holiday Inn Express in Custer.   Close to Rushmore,  Custer state Park and Crazy Horse.   Easy to go back at dark from your hotel.   Crazy Horse has a night light  show too.   Included in the price of admission to the Crazy Horse Museum /site.  

Stop and see the metal statue  of the Indian girl.   Chamberlin, SD.   Just before crossing the Missouri river.  

Avoid Wall Drug...  Unless,  you just think you HAVE to experience it.