Due to the volcanic nature of the island of Japan, there are numerous onsen or hot springs scattered throughout the country. Throughout the countryside, many ryokan own land with their own hot springs. Staying at one of these 'hot springs ryokan' is a recommended experience for any traveler.
There are many different types of hot springs in Japan with different minerals dissolved in the waters. The different minerals are said to have different health benefits, though all hot springs are considered good for relaxing the mind and body.
Humans aren't the only ones in Japan to enjoy the onsen. Snow monkeys enjoy onsens in a few parts of Japan but most famously in Nagano because of the publicity received during the 1998 Winter Olympics.