Lots to see and do in the Black Hills. Rushmore, Crazy Horse, Harney Peak, Custer Park, the Bad Lands, Sheridan Lake, Deadwood, Keystone... Used to live there and is one of the only places I've lived that I would return to but we ended up in Idaho for a bunch of good reasons and we love it!
Cody is great too!
If you have time the mine in Butte is worth the trip if you have any interest in the history of the mining industry.
You're going to have a great vacation, I bet you will be figuring out how to relocate before you get home ;-)