Wyoming
If you want to get from I-90 to 1-25 and so on down to the south, or vice versa, you'll pass through Casper WY, which has the best barbecue north of the Mason-Dixon line, at Banjo Bob's, at the Poplar exit, eastward. There's a place to stay nearby, too. And the stretch just north of Casper has interesting weather, so if you're traveling in a chancy season, parking there and having barbecue until the snow stops is a good idea. Somewhere I have a photo of our faithful Subaru looking much like a cupcake, with ten inches of snow on top. And it melted the next day, giving us no trouble moving on.

Head on down to Cheyenne, and on down. It's a nice drive, past places you'll recognize. The sky is large, the land is varied, and don't take weather warnings cavalierly.
Or.....you can take a side trip through the Big Horn Mountains, up northerly and westerly.

They're gloriously rugged.
We also ran into an Event, the seasonal cattle drive, yes, a real one, that brings cattle off the national land back to winter pasture.

This went on for miles. I'm used to cattle and had no hesitation about going through at a sedate 5mph, pausing for the confused ones who love to walk the centerline. We had a pleasant conversation with one of the owners...they drive them with horses, of which I do have a picture, but this one sort of got the real feeling, which is casual and strung out. The last thing you want is to have them run, so they amble.
And our chosen road led, via the East Thumb gateway, to Yellowstone National Park.
It's suffered a bit from forest fires, a few years back, but the recovery is going on apace, and the hills once blackened have gone green again. September foliage is spectacular, and the late-season traffic is much less.

We arrived just 10 minutes before Old Faithful erupted. This is a wonderful sight, and it was right on time and very tall---not all eruptions are as high. Another geyser went off at the same time, up at the other end of the basin. It may be one of the most dangerous spots on Earth---see, the Yellowstone Hot Spot, on the internet---but it's a marvelous place.
More, at this season, the wildlife was out in force.
Otters slid down a bank into the river near East Thumb.

We saw moose, and mule deer, and various small skittery mice or chipmunks. But not a single bear. When I was here in '57, bears climbed all over your cars, and looked into your windows to see if you had any sandwiches to offer them, and crazy people got out and fed them. The wildlife people found that the increase of cars, people, food, and excitable bears was Not Good, and began to ban bear-feeding. One assumes the bears are up in the woods in September beginning to think about snug dens for the winter, and we saw not a sign of them.
Buffalo, however, wander the lodge grounds---and if you have never seen the inside of the lodge, go! And bring your camera. It's unique.
Our buffalo.

Never, never take them for tame. They're creatures of fairly small brain, and if they sudden realize you've taken a position between them and the rest of the herd, or their baby, you are lucky if you escape with your life. They aren't fierce---usually---but their momentary placidity as they graze or doze tempts people to make Very Dangerous Moves. Believe me, the result of startling a buffalo can be deadly, as an animal the size of a small car uses its very sharp little horns to toss a confusing obstacle out of its way. I've seen them hit a car in which two very silly tourists had taken shelter. The car did not fare well. The buffalo was unfazed, but decided two dents was enough. They'd gotten between mama and baby to get a picture, and mama made her opinions known, then trotted back to baby.
And the elk were out in force, being picture postcards on the hoof.

This was right at twilight when they come down to water. I do urge anyone who visits the park to moderate their speed, especially at dusk and dawn. This place is special, and animals have the right of way.
If you want a slice of the west the way it used to be, go to Yellowstone, and plan to stay several days.