Everybody Needs a Scrub and a Soak!
After a day, or more, on the road there is nothing quite like scrubbing off the road grime and soaking away the aches and pains at an Onsen. With Japan sitting atop a volcanic fissure, hot springs are plentiful and often offer up therapeutic qualities perfect for road weary bones.
There is a long history of Onsen in Japan and the architecture, atmosphere location often reflect that. It is worth taking a long tour just to enjoy the hot-spring Onsens
Prefecture by prefecture Onsen Guide – with descriptions but no phone numbers or addresses
Great Kanto region Onsen guide – all info including maps!
Everything others know about Onsen at this wiki here