Hoyoland offers mixed and gender separated bathing in a large muddy pond. It’s awesome. The mixed bathing is awesome, but don’t be a dirty old man. The water and the mud is the best part really. The bottom of the main bath is just this thick hot spring infused mud that you have to work your way through. It feels like something that would cost hundreds of dollars at a day spa in Europe. You can enjoy the mud baths and stay in the enclosed gender separated parts of the bath if you don’t want to go outside. The main bathing area is just like a normal sento and gender separated so you can wash off all the mud with no problem. In the main muddy baths water is opaque so no one can see anything that you do not knowingly expose.
This is not a bathing experience for the timid, but it is one that is absolutely unforgettable and so enjoyable. Very simple Japanese style rooms are available for ¥7,000 with dinner and breakfast per person. The rooms do not have en suite toilets and shower but do include 24 hour access to the main bathhouse and free access to the rest of the baths during their operating hours.
Update September 2018: Still one of the most amazing places in Japan. The feeling of the mud is just incredible. Do not be a creepy guy please.