I'll add the following comments:
- Deadwood during the annual Sturgis Motorcycle Rally is a treat. Must like bikes.
- the Little Bighorn Battlefield is an unheralded gem. I absolutely loved that place. If anyone is going though, you need to read up on the whole battle before you get there to fully appreciate it. It is so thoughtfully, respectfully laid out. I was very impressed.
- if you're going to Little Bighorn might as well drive up to Red Lodge, MT for the evening. Stay at The Pollard, right on the main street. It's a great old hotel.
- From Red Lodge, you can then take the Beartooth Highway into the NE entrance to Yellowstone. It's a fun, relatively easy switchback drive up to 11,000 feet and then back down the other side. Much easier drive than Going to the Sun Road.
SD, WY and MT are just amazing states.