Grand Canyon Caverns is an old-school Route 66 cave stop east of Peach Springs. The cave is dry, deep, and odd: instead of dripping formations, its interest is in the scale of the chambers, the desert geology, and the way the attraction has accumulated layers of roadside Americana around it.
It is not in Grand Canyon National Park, despite the name. Treat it as a separate Route 66 detour, useful if you want something cool and underground between long desert drives.