Kanaya Ryokan (金谷旅館) is a 150-year old traditional wooden ryokan located near Rendaiji station just north of the town of Shimoda in Izu Peninsula which also allows day use of the onsen.
It has been featured in guide books and television shows for its sennin-buro or “thousand person bath”—a large wooden bathtub into which the natural onsen water flows. Not only is it one of the largest bathtubs in the peninsula but also one of the deepest, the water coming up to abdomen when stood up.
In the men’s tub, you can also find metal statues of naked women and a turtle (if I recall correctly) lifted above the water, while in the corner there is another tub set at a milder temperature. The water from the larger tub flows directly outside into the open-air bath—a smaller tub into which a narrow shoot drops water from above.
Unusually, Kanaya Ryokan’s onsen allow allows for mixed bathing (called konyoku in Japanese). But unlike most onsen that have a konyoku area (it isn’t all that common), there isn’t a separate bath where swimsuit-clad men and women can bath together; at Kanaya the women are allowed into the men’s bathing area on the proviso that they cover themselves with a bath towel. The male and female bathing areas are joined by a small intermediary chamber to which only females are allowed the key. A little surprising if you didn’t know, but it’s one rule that makes the onsen popular with couples.
Saturdays and public holidays: 1,000 yen for adults, 500 yen for children. Weekdays: 800 yen for adults, 500 yen for children. A towel set is 200 yen and shampoo, body wash, and conditioner is 50 yen for a miniscule sachet (there is no shampoo, etc. in the onsen).