I say continue on I-90, eh

, and hit Mitchel. Few little tourest stuff there. Corn palace, ect. Head to Wall SD and take the Badlands loop. Alot of little stops west of the Missouri River to see at your leisure. Me personally I'd take the 300 miles to Sturgis/Rapid City. Should be able to get a room at better prices. Spend a day or to there and check things out. Then take Vanocker Canyon outta Sturgis all the way to Hill City. With a turn or two. Hit all the places north of Hill City and when your resv are in you can hit all the stuff around Hill City and south. Saves you from driving back and forth. Black Hills HD at exit 55 always has stuff going on.