Maenohara Onsen Sayano Yudokoro
This beautifully landscaped place is actually, miraculously, a hot springs resort right in the middle of Tokyo. Along with large public pools to swim around in, there's all manner of indoor and outdoor baths fed from the free-flowing spring, which is especially rich in sodium chloride. There are full-length tubs you can lie down in, others that look like giant pots, and super secluded hydromassage hot tubs that give you a lymphatic massage as you soak. Book an appointment for a hot stone treatment or akasuri-a vigorous full-body exfoliation-and stop by the restaurant afterward for the most delicious buckwheat noodles.