We did an organized ride in the area last Labor Day. We stayed in Custer, SD & did daily loop rides. Custer State Park is a very scenic ride. We also did a loop through the Black Hills that took us past Mt Rushmore. There are many "pig-tail" bridges and tunnels. Lots of up & down. Lots of wildlife (bison, antelope, etc). There was very little auto traffic on Labor Day weekend (lots of Harleys however). I think we stayed in a near-new Holiday Inn Express in Custer that was quite nice and reasonable.
The Black Hills are aptly named. While there are no real mountains, there are a lot of hills. We had not trained adequately and the loop past Mt Rushmore we found quite challenging. I seem to recall it was 70 - 80 miles.
There is also a very popular route called the Mickelson Trail. It is a rails-to trails route that goes for many miles through the area. We did not ride on it but it parallels the highway in several places and appears to be hard-packed dirt or gravel. I think if you look up Mickelson Trail, you should be able to find a map on line.
Good luck.