Just read an
article in the NY Times about hidden natural hot springs in Akita. It was almost four years ago that I went to Akita's Lake Tazawa and had my first wonderful onsen experience.
In addition to food, onsen is really one the things I like most about Japan. Few things are more relaxing than soaking butt-naked in a spring, even better when it's outdoor. My favorite is
Sukawa onsen on one of the peaks of Mt. Kurikoma, with its milky spring water in an outdoor pool.