Hirayu Onsen is a mountain town which has a number of inns and a bit of skiing nearby. The waters here are rusty and iron-laden, and at Okuhida Onsen there's a wonderful wooden bath house with high ceilings and a charming pair of uchiburo with signs that read "Less Hot" and "More Hot." They're both pretty toasty, but great for anyone wanting to soak sore muscles or just let the stresses of the day slip away. If you get too hot, step outside to the expansive rotenburo, which has a number of nice takinoyu (waterfall baths) on wooden sluices. You can watch maple leaves float on the surface as the hot water cascades over your back and shoulders. Ahhh...a great soak, for sure.
Getting here's not easy -- easiest with a car. Buses run fairly infrequently so you'll want to budget an overnight or weekend if you're not in a rentacar. For those who are driving (or who have a chauffeur!) Hirayu Onsen makes a nice higaeri stop on your way to a larger place like Kusatsu or Takasaki. There's not a lot to do here other than dip, so you may want to skip it if you like night life or...any kind of life after sundown. But for those just wanting a spot to get away from it all, humble Hirayu Onsen offers some great soaks in water that's plenty hot enough to please.