Fort Union near Williston, ND is an interesting stop that sounds like it's on your loop if you're traveling between Glacier and the Black Hills. It was the Wall Street of the fur industry in the 1820s-1830s. Catch the Teddy Roosevelt museum (and park) near Medora on the way. And don't miss Devil's Tower in northeast Wyoming. The South Dakota Badlands is worth a detour as well.

I love driving out in the Western Dakotas and eastern Wyoming/Montana. It's scenic, but you can really cruise.