Shirahone is a well known hot spring area in the Alps, near Kamikochi. The milky white water is supposed to be good for a staggering number of illnesses (legend has it that spending three days in Shirahone will keep any colds away for three years), but either way it will warm your body all the way through and leave you feeling completely refreshed.
There are a large number of hotels here, many of which accept non-resident bathers during the day (usually between 10:30 and 14:00), such as Awanoyu.
The drive here takes a while, but rewards you with pretty mountain views and quiet wooded slopes. The village itself has a few gift shops where you can load up with local mountain vegetables in myriad disguises! The road may be closed after heavy snow or rain and the only cafe and some inns are closed in winter. Check ahead.