Spa LaQua

In many ways, LaQua feels more like a 'super sento' than an onsen. This sprawling bathhouse inside the Tokyo Dome City complex uses sodium-chloride-enriched hot spring water drawn from 1,700m underground, which is said to help circulation and relieve muscle ache, neuralgia and arthritis. In addition to an open-air rotenburo and foaming massage bath, the complex has three types of sauna, including an unusual rock salt one that's supposed to be good for improving metabolism. Once you're done bathing, you can take advantage of a bewildering array of beauty treatments, from traditional Thai and Hawaiian massages to aromatherapy and Korean body scrubs, or check out the various high-tech saunas in the separate Healing Baden zone. Spa LaQua is open all night, though note that there's a ¥1,890 surcharge if you stay past 1am.