Natural hot springs, onsen (温泉) are the ideal place to relax!
Japan is a land of volcanos, and therefore abounds in hot springs, which attract inhabitants of the cities during the holiday season. 
Although many onsen are also known for their healing power, their main characteristic is to be extremely relaxing.
Most often, hot water is gushing out from the depths of the earth, and is redirected to indoor baths fitted in ryokan (typical japanese hotels). Many hotels also provide an outdoor bath (rotemburo 露天風呂). It’s really impressing: just imaging to plunge into water at 45 degree (C) in front of the snow-covered nature… divine! But be careful not to stay too long, because of the temperature gap with the outside air.
Note: in Japanese, their is a specific word to designate hot water: _o-yu (お湯), by opposition with (cold) water, mizu (水, the one you’ll order in a restaurant). It is this same “honorable” hot water that is used for tea._