Yachigashira Onsen is a local public bathhouse near the base of Mount Hakodate, known for its reddish-brown mineral water. It is not a resort spa; the appeal is the everyday Japanese onsen experience in a city better known for night views, seafood, and historic streets.
After walking Motomachi, the foreign cemetery area, or the slopes around the mountain, the baths make a satisfying low-key stop. Bring the usual onsen etiquette: wash thoroughly before entering, keep towels out of the water, and expect a practical neighbourhood bath rather than tourist gloss.