1) Good hot spring source is key: In this picture you can see a milky colored hot spring which is repudiated for its healing properties. For me, milky colored water is stronger and more mineral rich than most hot springs. These types of hot springs contain some of the largest amounts of sulfur known in Japan. The only downside is that after soaking for a prolonged period of time, the sulfur stays on your body for days, sometimes even weeks, no matter how much you shower.
For those who want to take advantage of this type of hot spring’s curative properties, I recommend taking a few days off from work first. For me, I do think the benefits far out weigh the risk of stinking, though. Bathing in this type of water for 8 minutes could easily put you out for the day. Finding a really good milky colored source is not easy, though. Some establishments have been dishonest about their hot spring's mineral content, like in the case of Shirahone Onsen in Nagano a few years back where it was found that the owner had been dumping white powder into the baths for better color affects.
A good idea is to look for the “onsen association” sign at the front near the entrance or near the front desk, this way you know the onsen is being inspected on a regular basis. 2) Second thing I look for is the design of the onsen. In this photo you can see lots of wood. Wood enhances the onsen experience because wood, over time, releases its own minerals into the water. Wood and water are natural and they complement each other, and so naturally the wooded onsen bath is ideal. I am not a fan of fiber glassed nor tiled onsen. I think tile takes away from the onsen’s healing potential. Tile is the lazy man’s excuse for making it easier to clean their tubs and with Fiber glass you just poor chemicals and then scrub.
3) Remote location: I love an onsen that’s located deep in nature far away from major cities where the only way to get there is by car and a navi. Places like Kusatsu and Izu are too touristy for my taste. I love a place that’s rustic and isolated and not very well known. Final note: This onsen is located in Naruko in Miyagi Prefecture. Having been to this prefecture over a dozen times I have never written about it. I feel that Naruko is largely unspoiled and should remain that way, but like Kusatsu it's only matter of years before it becomes popular. This post was added to the Japan Blog Matsuri entitled " Japan how-tos" hosted by Nihongo Up.