Hi! Thanks for your comment. I fell in love with Tohoku since I was there in winter. I tried the second visit in Summer and it was awesome.
I have tried mixed open air bath for the first time in Takaragawa onsen, Gunma. It’s early spring in April, so there were not much crowd in onsen. But in Takaragawa onsen, they allow women to put on the ryokan’s towel while bathing. So it’s a good idea to try the first mixed bath here^^