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.Traffic most likely won't be as bad as it's late September and school being back in session. There is so much to see in that area. Mount Rushmore, Crazy Horse (only see it from the road, no reason to pay) Devils Tower, Custer State Park, The Badlands and of course one of the worlds greatest tourist traps, Wall Drug.
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.