on your return trip, Highway 2 across Washington is very scenic.
You can then pick up I-90 at Spokane, and follow the interstate to Sturgis.
We took 2 across on our way home a couple of years ago, it was quite enlightening
Some areas were as desolate as what you see going to Alaska, with all the beauty as well.
Other areas looked like a scene out of a Louis L'Amour novel.
Sounds like a great trip! keep us posted and put up some pix