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
The Japanese love their open spa/baths. There are many around Tokyo and all over Japan in fact. You can pop in for just an hour, get clean, relax, and get a massage if you wish. Perfect before a long flight. If you have a bit longer, why not take a train to Nikko, about two hours away, where you can relax in the Japanese alps!
There is a very nice 'onsen' in Narita town, excellent if you have a layover or if you have extra time after you've landed and before a business meeting. Nothing better than an invigorating shower followed by a relaxing hot soak after a long plane ride. The standard, traditional bath (all I've used) is only $10 or so, though they also have full spa services. Like most onsen, they also have a cafe serving food.
www.yamatonoyu.com/eng/
It's in Narita but perhaps getting a cab from Narita station would be easiest. It's not the gaudy 'water park' near the station; Yamata no Yu is further away - print off the map from their website and hand it to the driver.
Search Been there