Thanks, weegiewife!
And, of course, a big thank you to LTNphobia ^^^
As mentioned in other threads, I had some friends in Tokyo during the first few days and was able to use their hotel bathroom. We all went together to Mikoku Onsen for my first visit and it was wonderful! There was a bath where the water was only 28C (a little cold, but tolerable, great for cooling off in), another where the thermometer said 40C (perhaps it was 39C) – this was really nice, although I couldn’t stay too long in there. However, as the 28C bath was next to it I found it really easy to regulate my temperature.
Next to this was the fiery 50C bath. I didn’t see anybody use this one!
All of these were filled with natural black onsen water. I got remarks about how nice my skin felt after bathing here, very noticeably softer.
Also there was another bath at abut 43C with a green tinge, as well as another non onsen large bath with lots of powerful massage features. On one visit it was around 39C and I made good use of the shoulder pummelling waterfalls, on another it was hotter, around 41C, just over my threshold.
I came here four times and absolutely loved it, the ladies who frequent the baths were totally adorable.
The nearest sento to me in Yoyogi was unsuitable – not only was the changing and bath areas fully overlooked by a papa-san (nothing wrong with that, just unexpected, and a little unnerving), the changing area smelled bad and the only bath was 46C. Atmosphere was also completely different, lots of young women who used it pragmatically for similar reasons to me… as a place purely to wash and scrub, I only saw one person actually enter the bath, and she only went up to her thighs. What a shame it is this sento that has survived and that the other one (which I used to really enjoy) closed down.