スパワールド
A massive multi-floor onsen complex in Shinsekai, themed by continent — one floor does Asian-style baths, another does Roman, and they swap the men's and women's floors on a rotation. Open 24 hours, and you can sleep in the recliner lounges overnight, though they charge extra if you stay past 9am.
Open 24 hours; overnight possible but there's a surcharge past 9am. · Bring flip flops — you move between floors. · Heads up: Traditional onsen rules: nudity in the baths, and tattoo restrictions may apply.