Bangkok Onsen

Bathing, not massage. That's the thing to understand before you go. This is a Japanese onsen transplanted onto Sukhumvit Road in Khlong Toei, built around communal hot water and the ritual that goes with it, rather than the oil-and-treatment-menu routine you get at most Bangkok spas. If you want somebody to knead your shoulders for an hour, this isn't your place. If you want to sit in hot water until the city drops away, it very much is.

The opening hours tell you who it's really for. Doors open at 16:00 and stay open until 01:00, seven days a week, which makes it an evening thing, a long soak after work, after a flight, or after a day of walking yourself flat. I'd go late rather than early. And expect onsen etiquette, meaning a thorough rinse before you get anywhere near the shared water.