Msocko3 is right, Needles Highway and Custer State Park are some great rides. Just watch for critters in Custer State Park. There are no fences to contain buffalo, elk, antelope and mountain sheep and they can get very close to the road. Spearfish Canyon is another great ride. Do the pigtail bridge near Hill City for a unique experience. Our local HOG group does a 3 day ride down there every Father's Day weekend in June. About 50 to 60 total go there taking a different route each year from Jamestown to Sturgis first and then staying at the Golden Hills Resort (now the Day's Inn) in Leeds.
The last time I went to Sturgis for the rally was back in the 70's when I was a bit younger and single. Much wilder back then but now the majority of attendees are 50's or so.
Have a great trip!