The US is so big and so varied, I believe you can find some place great to travel without large crowds. The weather, however, is another thing altogether. Extreme heat everywhere will impact the enjoyment of our trips. That being said, I have traveled throughout California extensively, and throughout much of the Southwest and Northwest. There are many smaller towns in California where you can relax and relish the environment, be it seashore, mountains or desert.
New Mexico has more to see than the plaza of Santa Fe (as great as Santa Fe is!)...there is White Sands Natl. Monument, the Very Large Array, Chimayo, Chaco Canyon, Los Alamos, the Indian Cultural Center in Albuquerque, the balloon museum there.....this is just part of the list where you can go without the inundation of tourists. They will be there, but not in droves.
In Arizona, Tucson is between the two parts of the striking Saguaro Natl. Park, and you can visit some fine wineries south of town, plus a beautiful Spanish mission and art colonies. Tucson also has a great air museum. There are interesting towns like Bisbee and Jerome. Flagstaff has the Lowell Observatory, and Phoenix, though full of snowbirds in the winter, has a wonderful botanical garden, plus the Taliesen West house of Frank Lloyd Wright. Arizona is more than the Grand Canyon. (Meteor Crater and Grand Canyon Caverns don't have the crowds of the Grand Canyon). And don't forget Tuzigoot, the Petrified Forest, the Painted Desert, and Montezuma's Castle. All are less well-known and consequently less crowded, but still worth visiting!
In the Four Corners region, you can not visit the big national parks (Zion, Bryce, Capitol Reef, Arches) and go instead to Mesa Verde N/P, Monument Valley and hop over to Durango, Colorado for fishing and rodeos and rock hound gatherings, or a scenic railroad jaunt.
I have bragged about California in older posts, but needless to say, we have a huge variety of things to do and see, major and minor, popular and less well-known. I won't begin to list them, or the sights in Oregon or Washington. Another post, perhaps.
Get behind the wheel if you can and head off on a road trip!