I'll be staying near Tokyo in late December and want to stay at a monkey sauna resort. Where might I be able to do this?
That should be Jigokudani Yaen-koen in Nagano. You can get there about 2 hours and a half by the train from Tokyo. The best season is between December and March, so you will be there for the best season. http://en.jigokudani-yaenkoen.co.jp/