Mount Rushmore was the entire reason that we included South Dakota in our road trip. It is more awesome, in person, than I ever imagined. Equally magnificent is Custer State Park, whose granite spires totally blew us away.
Spearfish Canyon is definitely scenic. The narrow 1,00-foot limestone walls, strong-flowing adjacent creek, countless trees, and three waterfalls make for quite a drive. And you will learn an interesting factoid.
We spent the morning hiking Mammoth Hot Springs Upper and Lower Terraces to fully see things. It is different than the other thermal areas of the park because hot water rises through limestone. It is like being on another planet.