Hi John Kun,
Both Gifu and the northern part of Tohoku offer great onsen! The problem is that those are, as you know, isolated locations, and it is usually not possible to reach such onsen by train.
If you can consider renting a car, you probably would have much better opportunities to visit nice onsen. If you stay in those prefectures, even a day trip by car would allow you to reach really wild onsen...
For example, close to the village of Shirakawago (with the old, traditionnal houses with sharp roofs found in Gifu-ken), there is an interesting onsen called
Oyadani no yu, which is located on the Hakusan Super-Rindo road, in Ishikawa-ken. It is easy to go there by car, but there is no train - there is even no village around. This free onsen is located just next to a waterfall, and is in a really scenic area.
You mentionned Shinhodaka, which is a great place to soak in onsen. Most of the onsen there are large mixed baths, with nice views on the mountains. As for example
Karukaya, which I visited last year. However again, there is no train station around, the closest one being Takayama... about 1 hour by car away.
In Aomori-ken - which is probably as wild as you can go on the main island - the area around Towadako lake has great onsen in old ryokan. The most famous is
Sukayu Onsen, with its 1000人風呂 (1000 persons bath).
source :
http://www1.odn.ne.jp/~sukayu/img/daiyoku.jpg
In the Kuroishi Onsen area (north-west of the lake), there is a great ryokan in
Aoni Onsen. In fact, the ryokan is called Lamp no Yado, and there is no really point of giving a different name to the location, as there is absolutely nothing around: the ryokan is isolated in the middle of a forrest, next to a river. In winter, it can not even be reached by private cars. For a truely unique onsen experience, this might be the place.
For those places in Aomori-ken, there is again no train available, although buses might be used. They however are infrequent, but if you plan your trip carefully, it might be possible - especially if you overnight in one of the ryokan.
source :
http://onsen.secret-japan.com
A little to the south, close to Tazawako lake in Aomori-ken, there are also several famous onsen, like
Tamagawa Onsen (the most acidic onsen in Japan) and
Tsuru no Yu (Nyoto Onsen). Those are nice onsen, however, they tend to be crowded, especially during holidays.
Tamagawa onsen - source :
http://onsen.secret-japan.com
If you do not want to drive, there are still possibilities in the Kanto area. For example, Kusatsu could be a great destination. This famous onsen town in Gunma-ken can be reached by shinkansen followed by a car trip. So it Manza, which at 1800 meters altitude is one of the highest in Japan.
Since you read Japanese, you might enjoy this web site called
Hotsprings near Stations, and which offers lots of information on onsen that can be reached by train.
Or have a look at our onsen pages :
http://www.secret-japan.com/onsen
Hope to read from you after you come back
sanji
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