Drove through the Black Hills region of South Dakota. I can recommend 87 through Custer State Park as one excellent Miata road. Incredible scenic and full of hairpin turns. Even with Memorial day traffic, a few tactfully placed roadside stops allowed me to have the road mostly to myself. You cannot go all out; the road is narrow enough to find many SUV vehicles on your side of the road. (SUV vehicles must stay 6 feet from any dirt at the side of a road, or they may fall over due to their high center of gravity.) But still fun.
Also, there are a couple of tunnels that are just wide enough for a Miata. I had to wait for a chartered tour bus stopped in a tunnel. It seemed to me that he was about one or two inches away from the uneven walls at both sides. Could not see the top clearance. I think he had stopped to see how much space he had on all sides. Unfortunately, he got away when I tried to take a picture.
Pictures (click for larger size):