Hotel Nakanoshima is a hot spring hotel buit on its own island. You have to take a boat to get to the hotel. In the hotel there are all these tunnles and hallways connecting the various rooms, baths and other areas. It seriously feels like the secret lair of a James Bond villan. Day use is available but I recommend spending the night. The baths have a strong sulphur smell and all have amazing views of the mainland across the harbor. Highly recommended.

Update January 2020: No more day use option.