Glacier to Black Hills is a lot of open country, easy driving (except maybe for wind), but not a lot to do or see. Similarly Black Hills to Denver. As the Black Hills themselves - if you come from Chicago they are the first mountains. Coming from the west, you've seen higher ones.
If you head west, consider taking US2 and WA20. I90 through Idaho passes through an historic mining district, with towns like Wallace (until the 1990s the freeway passed through main street). The Washington part of I90 is not scenic until you cross the Columbia.
If you take WA20 across, consider taking it all the way to Port Townsend (includes a ferry ride), and pick up US101 there - unless you want to see Seattle itself.
And further south you have decide whether you want freeway and Portland, or more coast. You could take 101 all the way.
Unless you want to see San Francisco, consider taking CA20 from US101 to I5.
There's another option - south through Idaho (US93 or US95), west across Oregon (US20/26), and then sample the Oregon Cascades (anything between US97 and I5).
