Don't forget one of the most under visited parks in the NP system, Wind Cave. It is literally right next to Custer SP. Also the mammoth site in Hot Springs is kind of cool. A hike from Sylvan Lake up to Black Elk (formerly Harney) Peak is fun but make sure you are on top of the weather. It's an only mildly strenuous 7 mile (roundtrip) hike. On the way out or back take a pit stop in Mitchell and see the corn palace.