Ryugo
Visited Ryusenji No Yu, which is on the list from the pinned message. First of all, in the morning (we got to the onsen around 10:30 on December 28) there were surprisingly few people and the parking lot was half-empty. Glad I took the chance. Secondly, this place was clearly designed by a severely overworked salaryman (and that's a plus), because at every corner it's basically saying: "Lie down, take a nap, don't rush." There are genuinely lots of places to sleep: a big tatami area in the center, ganbanyoku, a library with sofas, sleeping capsules, lounge chairs on the terrace, a massage room... Basically, if you just need pure relaxation with minimal movement, it's all right here. Naturally, there's a proper cafeteria, and you can also book a massage on site. I can't write anything clever about the water since I'm no expert, but it has exactly what I love: a large variety of different baths indoors and outdoors and a couple of saunas. There's no really hot water though, max was around 41-something degrees I think. And there are 4 or 5 ganbanyoku, one exclusively for women, all the rest for everyone. Bottom line, my wife and I absolutely loved it. It reminds me of the general vibe of Manten No Yu (https://maps.app.goo.gl/L8pxV76VM9udJBEj6), which I'd been to several times before, but much bigger.