Kodai no Yu

It's a bit of a hike to get there, but this spa complex in Shin-Koiwa is one of Tokyo's biggest onsen. The three-floor Kodai no Yu looks more like a 'super sento' at first glance, but its baths actually use onsen hot-spring water, drawn from deep below Tokyo's surface. The baths on offer include outdoor rotenburo and hinoki cypress tubs. Once you're done wallowing in the steam sauna, you can go and get a massage or Korean body scrub, then retire to the fourth floor restaurant for some (fairly rudimentary) Japanese food.