Hwy 550 in Colorado, from Ouray to Silverton.
Hwy 12 in Utah, stop and take the hike to Calf Creek Falls.
Hwy 84 from Abiquiu to Pagosa Springs
All the small routes through the Sierra Nevada which I think are best east to west, 120, 108, 4, 88 and 49. 120 is the most famous but I like 108 it's less crowded.
On the interstate IS15 through the Virgin River Gorge can be surreal and even disorienting in the right light.
The Oregon Coast is fantastic, I've done it 3 times and I'm anxious to go back.
One of my favorite adventure drives was Tierra Amarilla NM to Chaco Canyon, through the Jicarilla Nation and the Navajo Nation. 95, 550 to 57. Most unpaved and it's really hard to tell what century you might be in.