Unzen Yokayu Public Spa

From the main indoor bath area there's an exit to a small balcony outside with an additional single-person bath. The color is slightly milky, no particular smell, water temperature is a bit above average, good ventilation - even a bit chilly until you get into the bath. An old-school, classic onsen, and there was indeed an elderly regular present. Whether you like this type depends on personal taste - I liked it. Price is 700 yen with towel rental, regular soap (i.e., bar soap, not liquid). Opening hours are decent, listed on Google as until 21:00, I think that's accurate.