On your return from Yellowstone you might want to consider taking the northern route of Hwy 2 into the Upper Peninsula of Michigan and continue into Lower Michigan. Plan a stop around Traverse City, Michigan and tour Sleeping Bear Dunes Nat'l Lakeshore. I think you'd be very impressed with the whole area. The Dunes and the park are huge and awesome. The area small towns are fun to poke around and there are wineries in the area. It's absolutely gorgeous.
After crossing the Mackinac Bridge you could do a day tour on Mackinac Island by taking the ferry. No vehicles are allowed on the island but there are ways to get around - mainly by renting a bike. It's a fun day.
Then you can skip Indiana Dunes and head back home. Have fun!