Funaoka Onsen is a public bathhouse, not a resort spa, and that is why it is worth seeking out. The building is famous for its carved wooden panels, decorative tiles, and old-fashioned atmosphere, with baths that feel woven into neighbourhood life rather than packaged for visitors.
It is a good introduction to Kyoto's sento culture if you are willing to follow the rules carefully: wash before entering the baths, keep towels out of the water, and be calm about the nudity. Go late afternoon or evening, after a day of temples in northern Kyoto.