I drove to Rapid City and back last fall, by myself with my then 2 year old. It was really a fairly easy drive. I'd say closer to 6 1/2 hours. It's not particularly interesting, but it's pretty easy. We stopped for lunch on the way up, as well as a couple of rest stops to let the toddler run around a bit. There are some interesting side trips, like the Oregon Trail ruts near Guernsey, Wyoming, and the Mammoth Site in Hot Springs, SD. Mt Rushmore is well worth the trip, although personally, I can skip Crazy Horse, I was not impressed. Some cool stuff in Rapid City, too...the Dinosaur Park (free) and Bear Country, USA (not free, but very cool).