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, not to mention one of the few that stays open all night long. 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, as well as family rooms that can be rented out. 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. If you stay there all night, there are communal (albeit gender-segregated) 'relax rooms' on the 3rd floor, which some people use as a cheaper alternative to getting a hotel – though note that a ¥1,575 surcharge applies if you stay past 1am.