There will be no nudie pictures in this post, don't worry!
Onsens are Japanese hot springs or spas - the big difference from what we're used to in this part of the world is that you are required not to wear any clothes. No bikinis, no knickers, no bra. No need even to modestly hold a towel in front of you!
Japanese onsens are said to be good for you, a bit like taking the waters, as the hot waters are of volcanic origin and contain a variety of minerals. I didn't feel different. I didn't glow. But it was fun, and very relaxing. Although, careful if you suffer from high blood pressure. Don't stay in the hot water for too long.
One of the best things about onsens is that they often have both an indoor pool and outdoor baths. So you can experience the coolness of the air and the heat of the water at the same time.
Onsens nowadays are segregated, so you can't go in with your husband, unless the place you go to has private baths. Which our ryokan in Hida-Furukawa, Yatsusankan, did. So, now that I was an expert, I got BB to try it out too.