I agree with all the other suggestions, but I would add one more. We loved the Grand Tetons actually in some ways more than Yellowstone. We spent a week there hiking and would live to go back.
If you go to Rapid City, check out Rafter J as a potential campground. Wonderful campground and great hosts. Our old truck acting up there and we ended up staying longer than planned. They went out of their way to accommodate us. If you spend time in South Dakota, you may want to check out the Corn Palace in Mitchell SD. Good for a couple hours.
