The Agua Fria National Monument is a hidden gem in the Southwest along The Great Western Trail. It has hundreds of miles of dirt roads through valleys and along the fringes of cliffs to test out your four wheel drive and nerves on. This area also boasts ridiculously gorgeous scenery, ruins to explore, and tons of wildlife such as grey fox, bobcat, mountain lion, wild horses, the occasional herds of longhorn cattle stampedes, the standard reptiles of the SW and many many more species of varying orders.
This area is definitely worth checking out if you have a vehicle with decent clearance that can take the steep grades in spots.
Just please keep it beautiful for future generations.

P.S. This National Monument really needs to get added to the list. If pictures are needed, I have hundreds, none less than 2MB though.