Yunomori Onsen & Spa Sathorn 10

Bangkok doesn't do many real onsens, which is why this one on a Si Lom side street keeps filling up. The hot spring baths are the point. Spa treatments sit alongside them, but if you're only here for a massage you're missing what makes the place worth the trip. Come for the soak, add the treatment after.

It's not a tourist-facing wellness package with lemongrass and a gong. Japanese bathing etiquette applies, which means a thorough shower before you get in the water, and a general expectation that you'll keep your voice down. People who like that will love it. People who want a chatty spa afternoon with friends should go elsewhere. What comes up again and again is how clean the facilities are kept and how carefully the staff run the place, which matters more in a shared bath than almost anywhere else.

My advice is to go late. It stays open until midnight, and the last couple of hours are calmer than a Saturday afternoon, when it gets busy.