Thank you all for your replies. Just to give you a little more information about our itinerary...
On July 7th we will be driving to the Devil's Tower area from Roosevelt Lodge in Yellowstone and staying overnight there. Depending on when we arrive in the Devil's Tower area, I was planning to explore the area (hike, picnic, etc) on July 7th or July 8th. On July 8th we will drive to the Mt. Rushmore area and stay overnight there. Again, depending on the arrival time, I was planning to see Mt. Rushmore on July 8th or the morning of July 9th.
What I have to plan now is starting the drive back to Salt Lake City. We will fly out of Salt Lake City on July 11th. So, I was planning to drive roughly halfway. Of course, if there is a recommended stop, I can change that to be longer/shorter for either day. On July 10th we would drive to Salt Lake City and then fly out the morning of the 11th.
Given that, thank you in advance for any other suggestions...