The Black Hills area has a lot to offer and many "must sees". You did not mention how many days you are planning to be in South Dakota or what your interests are. Knowing this info will help TA advisors help you.
What are your expectations of Deadwood? Are you planning on gambling or are you more interested in the historical aspect of Deadwood? Many are disappointed with Deadwood because they are expecting the "old west" town with boardwalks much like what was depicted in the HBO series "Deadwood". The buildings are old and many very historic, but the original buildings burned in a huge fire many, many years ago. Most of these old buildings house casinos and some hotel/casinos. I am not trying to talk you out of Deadwood but bring this up regarding lodging. If you plan to gamble then you will probably want to lodge there. If you are interested only in the historical aspects of Deadwood, allocate a day of sightseeing and consider staying in a more centralized location in the Black Hills, such as the Keystone/Hill City area, only a 45 minute drive. These locations would put you in a closer vicinity to Mt. Rushmore and most of the Black Hills attractions. A "must see" while in the Deadwood area is Spearfish Canyon, a gorgeous canyon with several beautiful waterfalls.
I agree with wings1980 regarding Custer State Park, although I feel you could easily allocate at least 2 days to seeing the park. A state park, many feel the park should have national status.
Let us know your time frame and interests and we can give you more advice.