Visit the 'Museum of the Sewerage'
https://www.youtube.com/watch?v=Q2g5H5Wbr9Q
Not satisfied with their tour of the toilets at the Toto Tokyo Center Showroom, the folks behind Only in Japan have now decided to head out to the so-called "Museum of the Sewerage" in the city of Kodaira in western Tokyo. That's right: it's a sewage museum.
You can spend the day learning about Tokyo's sewer system, and even go five levels down to stand on a viewing platform right inside the main sewage pipe for Kodaira—which smells about as good as you can imagine. But at completely free, you certainly can't complain about the price!