Some more in Colorado:
St. Elmo SW of Buena Vista. Graded gravel road primarily following an old railroad grade. You can continue on the road up toward Hancock for another 6 or so miles and there are various mining ruins along the road. Once you cross the creek, there's a trailhead where you can walk to the east portal of the Alpine Tunnel (about three miles).
Bonanza at the NW corner of the San Luis Valley. Graded gravel road but easy driving.
Gothic north of Crested Butte. Just a few buildings here, but the wildflowers in July and August are some of the best anywhere in the nation. Again, an easy graded gravel road.
Marble, CO. The marble quarry here supplied the stones for some of the buildings in D.C. Also do the loop through Redstone on your way here (it's a one lane street on the other side of the river from the highway). Some charcoal kilns right beside the highway. Paved road all the way to Marble.
There are lots of smaller ghost towns above Cripple Creek. Cripple Creek itself has legalized gambling which has kind of changed the complexion of the town. But as employees looked for housing, some other towns like Victor have been preserved and restored to an extent. Lots of paved roads up here and many of the others are well maintained gravel roads.
Same can be said for Blackhawk and Central City. Lots of smaller ghost towns in the surrounding hills.
All along US 550 between Ouray and Silverton there are mining ruins visible across the valley. There are also a few right along the highway like at the view area about 1 1/2 miles north of Red Mountain Pass. If you're serious about seeing lots of ghost towns and mining ruins, rent a jeep in Ouray or Silverton and take a day to explore those back roads. Pretty interesting stuff.
As boogie 4-wheel mentioned, head to Nevada to see a lot of ghost towns without the crowds around.