This historical museum run by the Tokyo Bureau of Waterworks has surprisingly interesting exhibitions on the history of the public water supply in the capital, spread out over two spacious floors.
Upstairs you can get a glimpse into daily life in urban Tokyo in the 1600s, with an exhibit showing reconstructed terrace houses grouped around a community well. You can also see some real wooden pipes that date back to the 1700s and were excavated in the Yurakucho area in the 1990s.
The downstairs exhibits chronicle the establishment of modern waterworks in Tokyo, showing off a replica of a public drinking fountain for horses and cutaway sections of gigantic metal pipes between 800mm and 2900mm in diameter. Signage is in Japanese but an English-language audio tour is available.