Niwa no Yu

Don't let the fact that it's located next to the dreary Toshimaen amusement park put you off: this is the nicest of Tokyo's mega-onsen bathing complexes. Housed in a Japanese garden designed by leading landscape architect Kenzo Kosugi, Niwa no Yu is divided into male and female onsen bathing areas, with a central pool, outdoor jacuzzis and Finnish-style sauna where couples can hang out together (bathing suits required, natch). The sodium chloride-rich hot spring waters are said to be good for relieving muscle pain and stiff joints, while the absence of screaming children (you have to be at least junior high school age to get in) guarantees maximum relaxation.