Regarding tour companies, we did a backroads tour two years ago and loved it if you are at all inclined to activity. They have mixtures of hiking and biking, but the guides, organization, lodging, and food were all great. You do as much or as little as you want.
Beyond that, a few thoughts:
1. Yellowstone. Yellowstone is indeed extraordinary and really warrants several days as there are some distinct regions (the lake area and old faithful, the east where you see the most wildlife, the falls, mammoth hot springs etc.) All amazing and in many ways must sees. BUT, they are really spread out, and for my taste, there is something about yellowstone that really makes it a "stay in your car" kind of park, which sorta defeats the purpose.
2. Glacier, on the other hand, is very much a "get out of your car" kinda place. GTSR is really cool (follow the plowing pics in early spring!), and we had a great time hiking in the east and riding horses in the west.
Eat huckleberries every chance you get. We actually stayed in Whitefish, which was far, but not too far, and provided a great variety of options for both lodging and dining.
3. Depending on what the border/covid situation is, if you go to Glacier, think seriously about taking a day or two and crossing the border into Waterton Lakes, which is the Canadian half of Glacier and is spectacularly beautiful while somewhat less crowded. The difference between the Canadian and US parks is interesting in that the us parks' infrastructure was built "as" infrastructure, whereas the Canadian parks' infrastructure was built as towns. As a result, the towns are better to stay in, and the parks are a little wilder.
4. While it's a fair bit to the east in Wyoming, our first national monument -- Devil's Tower -- is always cool.
5. I assume Utah is out of bounds, but Bryce and Zion are great.
6. If you have any interest in flora photography (as oppose to fauna), take a camera with a microlens. There are unbelievable wildflowers in mid summer.
7. Various pics from Alberta - Montana - Wyoming - Utah trips here:
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