Onsen (hot spring baths) are a great way to unwind after walking all over the 23 wards of Tokyo. The public bath in Japan originated back in 710-784 for religious purposes; but today most, if not all, modern homes have baths. During the after affects of World War II, the public bath houses saw resurgence due to practical reasons. Onsen is the diametric opposite of everything in normal, hectic day-to-day life. It's an opportunity to relax in an almost meditative way. One of the most magical experiences I've had is sitting an an outdoor Onsen with snow falling on my head.
There are two main types of public baths: Sento which are an ordinary heated water bathhouse, and Onsen which are geothermally heated beneath the ground erupting from the ground at a minimum of 25C and are required to have one or more of the 19 elements that naturally occur in hotspring waters. The Japanese generally prefer Onsen water, as they believe it has healing powers depending on its mineral properties and content. Onsens often provide several types of baths, differentiated by the presence of different minerals, and their properties. Japan is a volcanically active country and has thousands of Onsens scattered across the country. The presence of an onsen is often indicated on signs and maps by the symbol ♨ or the kanji 湯 (yu, meaning "hot water"). Or sometimes the simpler hiragana character ゆ.
The idea of communal bathing is likely to put-off a lot of foreigners, but I'd strongly recommend trying it at least once since it is a significant part of Japanese culture. It's also incredibly relaxing to enjoy hotsprings at an outdoor Onsen.
There are a few etiquitte rules to follow to be respectful. First, everything is considered unclean and would be an egregious bathing sin to bring anything into the water other than your body. You'll leave everything except a small wash-cloth in a locker outside the bathing area. (yes, everything. Wearing swimsuits is quite rightly forbidden) Once you enter the bathing area you'll see faucets, small stools and toiletries. In this area you have to get as completely clean as possible. The actual bath is not meant for cleansing, it is exclusively for soaking so be sure to be spotless clean before entering the water. This keeps the communal bath waters clean.
When you enter the bath, ease in without splashing or diving. Don't let your washcloth touch the water (you can fold it and place it on top of your head). Never let the towel drop into the water or wring the towel out into the water. Don't submerge your head or hair. If you have long hair, keep it up and out of the water.
Onsen are places to relax and meditate. Although the baths are usually quite silent, there is nothing wrong with talking quietly to friends. People in the same bath or pool may well start a conversation with you. However for the Japanese to initiate conversation with a strange foreigner is quite unusual.
One thing that people always ask me about is tattoos and Onsen. Tattoos are something of a taboo in Japan – thanks for the most part to their association with Japanese organised crime (the Yakuza). You will want to look out for large signs at the entrance and greeting area that will indicate if the bathhouse has a strict policy on tattoos. You stand a higher chance of being noticed or asked to leave if you are Asian and have tattoos, if they are of Yakuza symbols (dragons, eagles), or if they are large whole body or sleeve pieces. Don't let this discourage you! Many establishments don't care. With the oncoming 2020 Tokyo Olympics there is a pressure on businesses from the tourism board, and the government to be more lenient on tattoos, especially on foreigners.