If you are coming in from Rapid City, it is only a short detour to scoot across the border into Wyoming to visit Devil’s Tower.
When you leave, I would suggest driving along the Spearfish Canyon Scenic Byway (Route 14A) from Spearfish to Deadwood.
Once you arrive in Deadwood, if you have time, you can take a tour of a gold mine and do some gold panning at the Broken Boot Gold Mine.