We frequently vacation "out west" most often in late September or early October. So far, we have visited Rocky Mountain National Park, Zion, Capitol Reef, Arches, Canyonlands, Grand Canyon (North rim), Grand Teton, Yellowstone, and Glacier.
There are at least three ways to talk about accessibility: 1. getting to the park; 2. finding accomodations in or near the park, and, 3. seeing the attractions within the park.
RE #1: If you have never been out west, most Easterners are shocked to discover how big the American west is. Some of these parks are very remote, and not really what I would call easily accessible unless you are prepared to spend a lot of time driving a car. We usually drive (from Missouri), so I cannot comment on how easily one might be able to get to these parks from an airport.
RE #2: Some national parks have a town or small city not too far from the park where you can find hotel/motel accomodations or commercially operated campgrounds and restaurants. For example, it's 33 miles from Moab to Canyonlands National Park. Accomodations inside the National Parks -- such as lodges, cabins, and camping sites -- fill up every night during the peak season, so you will want to get reservations. There may be a camp store inside the park with very basic food items, but I suggest bringing a good supply of your own food into the park. Except for possibly Yosemite, where I've heard the bears are very good at getting to your food.
As for #3, once you get inside the National Parks, most of the major attractions are no more than a couple miles hike from the parking lot. But first you must find some place to park your car. Last time I visited Yellowstone (June 2018), the parking lots to the main attractions were all full, and we got so frustrated we just left the park without walking to any of the sights. If I had paid $20 to get in, I would have been pissed, but we have the senior pass, so admission was free.
Some parks offer a shuttle service or bus to the attractions, which may be preferable to trying to find a parking spot.
I love our National Parks, but so do MANY others. For me, the crowds almost ruin the experience. It is still possible to escape the crowds if one is willing and able to walk more than four or five miles. Later in the season, there is slightly less crowding, but if you wait too late they start closing the roads and campgrounds.
