Nicole,
Leaving Yellowstone (please spend at least 2 days there) the town of Cody has some good things to see such as the Buffalo Bill Historical Center, the Buffalo Bill Museum, and the Whitney Gallery of Western Art.
Then, just west of the Black Hills is Devil's Tower (remember "Close Encounters of the Third Kind"?). And, just in case you don't see lots of wildlife in Yellowstone, the wildlife loop in Custer State Park south of the Black Hills has buffalo and pronghorn antelope. Our son was just there and had to stop for herds of both to get off the road, but he didn't mind the wait.
Finally, the Wild Horse Sanctuary, south of Hot Springs, SD, is a wonderful place to walk among wild mustangs and see a few interesting petroglyphs as well.
Have a great trip.
Craig Sheumaker
co-author of the travel guide: America's Living History-The Early Years