Not sure how you will be departing Ft Robinson, but about 30 mi east on US20, just past Chadron is the Museum of the Fur Trade. It is chock full of everything related to the fur trade, Indian fur trading, the trading companies, etc. Extremely well organized and worth two or three hours to see. Built on the site of an old fur trading post.
http://www.furtrade.org/Also in your travels in the "hills" don't miss the Mammoth Site in Hot Springs, a rather quaint town with some neat red sandstone buildings.
http://mammothsite.com/YOU DO NOT WANT TO BE IN THE BLACK HILLS THE FIRST TWO WEEKS OF AUGUST!!! The Motorcycle Rally at Sturgis is Aug 3rd thru the 12th 2018, and the bikers begin touring as early as late July, however they are all gone within a day or so after the end of it. I arrived this year on a Tuesday in Spearfish just two days after it was over, and encountered no bikers at all. Generally attendance is somewhere around 400,000 and the Iron Mountain highway and the Needles Highway along with the rest of the Black Hills are favorite riding areas.
Others may disagree but visiting Rushmore, or simply riding along seeing the scenery with large crowds of bikers passing by and behind and in front of you is not my cup of tea.
Charles