Billings is the main town between GNP and Black Hills. Little Bighorn is in the area.
There are some grades and passes on US191, for example approaching Flaming Gorge dam, and between Duchesane and Price. Moab is a major destination on 191 (Arches, Canyon Lands). The drive from Moab south and then SW to Ouray (UT 49, CO90 etc) is nice. But looking further at your map is looks like you are thinking going south via Grand Junction (consider Colorado National Monument). Maybe you could go to Ouray via Grand Junction, and return to I70 via Moab. There are lot of routes and sights in SE Colorado and S Utah (Glen Canyon, Capitol Reef, Escalante, Bryce, Zion, Natural Bridges, etc).
