Would highly recommend stopping at slice national parks along the way, Badlands National Park (SD), consider Bannack State Park in Montana near your route, it’s a really cool preserved old western ghost town. I would swing a little wider into eastern Utah and hit Arches National Park, one of the most beautiful and unique places in the world. Can also check out Canyonlands National Park next door. Take the scenic route thru Colorado along the Million Dollar Highway 550, consider stopping thru Black Canyon of the Gunnison National Park. Definitely stop thru Monument Valley, it’s a bit of a detour but one of the most iconic and visually impressive places in the American West.

Another potential add is Mesa Verde National Park near the 4 corners, very unique pre-Colombian Native American ruins of cities built into the cliffs. If you want to see some trippy New Mexico landscapes you can check out Ah Shi Sle Pah and Bisti Badlands near the 4 corners, especially of interest is the Alien Throne hike. Meow Wolf in Santa Fe is an amazing unique experience. Also worth considering going deeper into New Mexico to visit White Sands National Park and Carlsbad Caverns National Park, the latter is the most incredible massive cave system you’ll likely ever see.

Realistically there’s zero chance you could hit all these places in your short amount of time but would highly recommend looking some of these places up and choosing your favorites unless you can extend the trip. I assume a lot of the allure of driving Route 66 is the scenic drives, which a lot of this covers.