Montana, North Dakota and Minnesota places of interest?

We will be traveling from northern Michigan to Denver Colorado the last two weeks of August to visit our son. After spending a week in Denver we will be heading to the Tetons and Yellowstone for three or four days camping at Fishing Bridge on Yellowstone Lake. We plan to return to Michigan around September 5 via I-94 through Montana, North Dakota and Duluth Minnesota returning through the upper Peninsula to our home base in northern Michigan.
Any suggestions on places to see while on our way home? Or any suggestions on good places to camp.
Thanks, Dave
Any suggestions on places to see while on our way home? Or any suggestions on good places to camp.
Thanks, Dave