Mt. Rushmore & Badlands-Must-See Suggestions

Lots of great suggestions above. There is so much to do in the Black Hills. Mount Rushmore is a must see. Custer State Park - ask about the location of the buffalo herd. Drive the Needles Highway, hike around Sylvan Lake, take a day trip to Devils Tower or a cave tour. Our favorite "touristy" things to do when our kids were younger were Reptile Gardens, Bear Country USA and Old MacDonalds Petting Zoo. There is an 1880s train that runs out of Hill City. Personally I would skip the Crazy Horse monument.
The Rafter J Bar Ranch is our favorite place to stay. Great location with spacious campsites. I believe they also have cabins.
Enjoy your trip!