Japan
Situated next to the Toshimaen theme park, Niwa-no-Yu Onsen (garden spa) is a relatively new super onsen, about 15 minutes from Ikebukero by train. Costing 2200yen for a ticket, you get towels, yukata, and even a free toothbrush and razor.
Inside is a selection of different flavour natural spring hot pools, sauna, steam room, cold pool, and then a lovely garden-set rotenburo outside.
Once you've bathed (usual caveats about Japanese bathing apply - shower first, so you're clean when you get in the pool) you can slip into your yukata and pop upstairs. A restaurant, bar, smoking terrace, and relaxation room await. The reclining chairs with the birdsong tweeting out of the speakers in the headrest are particularly good for a gentle sleep.
As far as I can tell there is no time limit on being there - the last visit we spent about 5 hours, bathing, sleeping, eating, bathing etc.
Super clean, very luxurious and not as gimmicky as some of the other themed onsen in Tokyo city. Worth a go if you can't get out to a countryside onsen and do it early in your trip, because after your first try you'll definitely want to do it all again.
A short walk from Toshimaen station, on the Oedo line or Seibu Ikebukero line. web-japan.org/tokyo/tips/onsen_tour/onsen_cont6.html
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