>>1452365In no particular order, the best I've found to date at least:
-Hoshi Onsen in Gunma
-Aoni Onsen in Aomori
-Yamamizuki in Kurokawa
-Hokuten no Oka in Abashiri (hotel, not ryokan though)
-Public rotenburo at Zao
-Tsurunoyu at Nyuuto Onsen
-Takaragawa in Gunma (accomodation bit aging though)