Soaking in a hot spring spa is regarded as one of the best ways to relieve stress in Japan.
Each hot spring has their own attraction.
Especially, an open-air bath is highly recommendable. It is wonderful in winter. (Now is the best season.) Because you feel both warm and cool at the same time. Soaking in the open-air bath feeling cool air on your face and seeing beautiful scenery can be a gorgeous experience.
Now more and more inns in hot spring resorts have a private open-air bath like a picture below. Then you can enjoy taking a bath privately in your group without caring about others.