Kaiō Natural Onsen, Imizu
A day spa beside the 'Kamon Park Shinminato' roadside station in Imizu, on Toyama Bay. Its spring is a dense chloride water said to rank among the richest in Japan in dissolved minerals, with several times the free carbon dioxide of shop-bought bath fizzers, so onsen enthusiasts know it as a genuine carbonated chloride spring. The tea-brown, salty, iron-rich water warms you deeply; there are indoor and open-air baths, a sauna and a cold plunge. What stands out is the long day: an early-morning bath from 05:00 through to midnight (23:00 on Sundays), with cheaper early and late rates. Regular admission is ¥970 for adults (¥1,020 weekends and holidays), ¥750/¥780 early or late, ¥520 for children. Closed Mondays (open if a holiday, then closed the next weekday) and January 1. Easy to pair with the Shinminato Bridge, Kaiwo Maru Park and seafood at the Shinminato Kittokito market; about 10 minutes by car from Kosugi IC or 20 minutes by community bus from JR Kosugi Station.