After hiking or motorcycle touring especially, I always plan my route so that I end up having a nice soak! It`s easy to do since hotsprings are seeping out of the ground throughout the country. I`m discovering new one`s all the time and they frequently are in fantastic settings, high up in the mountains, along riversides, on beaches and even in caves! Usually clothes are not permitted so bathing is often separated however it`s quite common to find some in the countryside that are co-ed....very friendly! .Here`s some of my favorites:

Ki Katsuura - this is an amazing place where you can soak in a cave over-looking the ocean. On windy days the waves crash right into the bath at the cave entrance which helps you cool off.  This hotspring has a strong sulphur content. Ocean view accomodation is also available.

http://www.hotelurashima.co.jp/hotspring/boukido.php

Yubara - definitely not for the shy. This hotspring is located in hot pools just below a large dam in the river. Bathing is co-ed and wide out in the open. Great place to make new friends!

http://www.yubara.com/higaeri/

Kurokawa - cozy, classical style hot spring town. Bathing is in pools (and caves) along the river. It`s worth treating yourself to a traditional Japanese inn here!

https://www.kurokawaonsen.or.jp/

Kinosaki - only a kilometer or so from the Japan Sea coast this hotspring town makes a great over-night trip from Kyoto. You can easily shuffle in a bath robe from one facility to the next, all having there own unique style. Plenty of accomodation is available and the tasty local beer and sake is definitely a bonus!

https://kinosaki-spa.gr.jp/