In the snug town of Aso at the foot of the eponymous volcano, Yumeoisou is a nice, old-style onsen ryokan with a number of kashikiriburo (private, reservable baths) as well as some atmospheric public baths that are popular. There's a lovely foot bath, too, if a real dip isn't possible.
Aso's transportation was damaged in the 2016 Kumamoto earthquake, but efforts are ongoing to repair damaged roads and bridges. Car, bus, and train are all possible.