A special version of
Onsen are called
Rotemburo, literally translated as "
open air bath". They can often be found near nice natural settings in
Japan (
hills,
forests,
ocean etc.) and at least some of their bath tubs are open air (with a roof to protect against
rain and snow).
Rotemburo are a great experience, especially during winter. Imagine that you soak your body in steamy hot spring water, a wet towel on your head as well as some hot sake in your stomach. The air around you is chilling cold and with the dim light coming from Japanese lanterns, the steam from the water is visible against the sky. You look above you and you see the stars and maybe the moon. Next to the bath tub some white, glowing snow. Nearby you can hear the ocean. It's gorgeous.
Another specialty of Onsen in Japan is that most of them belong to rather sophisticated Japanese inns (Ryokan), which offer typical tatami rooms and delicious Japanese meals. Soaking, drinking, eating, sleeping -- after three days, you will feel completely newborn.
The only drawback is that most Onsen and Rotemburo these days are strictly divided into tubs for male and female (although in the past, bath houses in Japan used to be mixed). For couples and mixed groups of friends, this is only half the fun.
If you look for mixed bath houses, ask for Konyoku Onsen or Konyoku Rotemburo. Some still exist. My favorite one is near the beach of Izu Hanto (Izu Peninsula).