Howdy folks..

In 2 weeks I'll be going on a 3 week trip thru the NW US (and SW Canada).
I'm pretty fine with the first leg from my starting point, Calgary, to Butte, MT (incl. Glacier/Waterton NP)

But still a bit unsure which route to take from Montana to Washington.

Going west from Montana, I could stick closer to the border and follow I-90 (not necessarily driving I-90 all the way) to Spokane. Then veer off to the North to go thru North Cascades NP on Rte. 20 (via Winthrop, WA) to Seattle.

An alternative routing would take me further South from Butte via Idaho Falls, Craters of the Moon NM, Boise, (Portland OR optional), Mt. Rainier, to Seattle. I think I could also visit some sites along the Oregon Trail if I chose these routing.

Time budget will be relatively flexible for either route. I think 3 nights on the road would be a minimum, 5-6 nights probably the maximum.

My interests:
Have a nice drive with some scenic or interesting intervals.
A few spots on the way to explore some history. Anything that could be tagged from "Old West", ghost towns to 1950s/60s Americana sure would interest me. Even if it's not some grand-scale tourist attraction.
Do some short hikes/trails (1-2 hrs).
Nature/ national parks are also very welcome, though I am very happy with the not-so-famous ones.

Any ideas which route would be "best" or what sights or options I am still missing.
Thanks in advance for any suggestions.

First, why Butte? Given your interests (history, interesting towns, etc.) I'd recommend Helena much more than Butte. Fascinating history, tons of wonderful buildings, a very cool place. Butte, bless it, has an awesome hole in the ground and some interesting (mostly depressing) history, but Helena has more going for it IMO.

From there, I would probably head south to Yellowstone and spend a day (at least) in the park. I'd see Mammoth Hot Springs and some of the geyser basins at a minimum - it will be crowded but still well worth it.

Then I'd head west and use I-84 to the Columbia Gorge (using the "historic" Gorge highway from Hood River to Portland. Then north on I-5 with a detour to Mt. St. Helens before arriving in Seattle.

This route would be best with the longer (5-6 day) time frame than with the shorter one. Butte/Helena to Seattle is one long driving day, so use that as your baseline.

Thanks!

Well, Butte.. mostly because of sentimental reasons, sort of, as the town and the old Finlen hotel played a role in one of my favorite movies. I kinda liked that somewhat ghosttown like look of Butte on the pics I have seen. And I want to see the mining museum.

But Helena will be another stop/ overnight on my way down from Alberta. So I won't miss the sights you mentioned.

I am somewhat reluctant to include Yellowstone, though. I am quite aware of the massive appeal of the park and the beauty. But I usually don't travel in high season and any accomodation I have checked in Jackson or the Western gateway towns are quite expensive. Not that that would amaze me.
But I think the park might just be a bit too crowded that time of the year.

Thanks for bringing up the alternative via Columbia Gorge. i did not have that one on the radar yet.