I loved Deadwood! I think you would enjoy spending a day there and going through all the old buildings and shops--like Saloon#10 where Wild Bill Hickok was shot and Kevin Costner has a casino there. I would also recommend the old cemetery. I just enjoy old towns...
I do agree with Jim that you would probably want to make your base near Hill City, to see Mount Rushmore and Crazy Horse. Some of the drives are so pretty and Custer State Park will offer you glimpses of buffalo and other wildlife. If you like this sort of thing, Bear Country USA is a drive through wildlife park where you can see bears, wolves, etc.
It's a beautiful part of the country and I'm sure you will enjoy!