Ryokans are traditional Japanese guesthouses. The most famous can be found in the Kansai area (Kyoto, Nara, Osaka) or in remote parts of the Japanese countryside.
Some of them also have onsens, hot sources, that are a common Japanese feature in this volcanic country. Last, all rooms (there are exceptions, so check before you reserve) are tatami-rooms so prepare to exercise your will to stay put during the whole meal. Futons are also ultra-thin, so beware !