Good job, George; Coal Mine Canyon is the answer.
This is not what you'd call a well-known attraction. The canyon itself is on the Hopi Reservation, but the rim, which includes the best viewpoints, is on the Navajo Reservation. The spot where most people go is very close to the highway, but because it's tribal land, away from the public right-of-way, you have to purchase a permit (available at the Visitors Center in Cameron) just to drive out to it. There's no infrastructure, no signage, no restrooms. Just a couple of concrete picnic tables, and a completely outrageous view of this amazing canyon that most people, including most Arizona residents, have never heard of. It's definitely worth the little bit of trouble, if only for the bragging rights. (And the photo op.)
The windmill is important because of the aforementioned lack of signage: you look for the windmill (the only visible windmill for miles around), so you'll know which dirt road you're supposed to turn on, about 15 miles east of Tuba City. Coal Mine Canyon is part of
Scenic Side Trip #15, which is a very cool route through the heart of the Hopi mesas, where the Hopi people have made their home for more than a thousand years.