Soak in black hot spring water at a local bath
Around Ota and Shinagawa in southern Tokyo, some sento, the neighborhood public baths, are filled with natural hot spring water that rises from underground as dark as strong black tea. It is called kuroyu, black water, and you can soak in it for the price of an ordinary bath without leaving the city. Bring a small towel or buy one at the counter, and check each bath's tattoo rule before you go.
A bathhouse founded in 1923 with two natural hot springs of its own: tea-black kuroyu and a golden-brown spring from deeper underground. Even the cold plunge pool is filled with black water, which the bath is known for. It is about 5 minutes on foot from Musashi-Koyama Station on the Tokyu Meguro Line.
A hot spring bathhouse about 5 minutes on foot from Kamata Station, with an indoor bath of deep black water and a half-open-air bath where you feel the outside air. It stays open until late at night, so you can stop by on the way back to your hotel.
A bathhouse from 1929 in a residential part of Kamata, where neighbors drop in for a soak in kuroyu, the black hot spring water typical of Ota ward. It is a plain local bath rather than a spa, so you share the everyday routine of the neighborhood. It is about 8 minutes on foot from Kamata Station and opens in the afternoon.
A day-trip hot spring a few minutes on foot from Zoshiki Station on the Keikyu Line, with a large bath of dark brown kuroyu. It is bigger than a neighborhood sento, with a sauna and an open-air bath as well. Zoshiki is on the line between Shinagawa and Yokohama.
A bathhouse next to Togoshi Ginza, a shopping street over a kilometer long lined with snack stalls, with baths of natural black water. The men's and women's sides swap from day to day, and each has its own design. Soak first, then walk the street for snacks.