You can enjoy snow field between December to April or May.
It is very cold, but pure white world with powder snow.
You can ski, and you can walk by snow-shoeing,or crampons.
Please check nice photo in my blog as follows. You can see Japanese
antelope, monkey, rabbit, deer, if you are lucky.
長野県・飯山
https://ameblo.jp/cx0293/entry-12433245392.html
If you have no time, it is enough to walk only one hour in North Yatsugatake.
長野県・北八ヶ岳