Anastasia
From Asakusa to the nearest station it's an hour by train, and from there their free shuttle runs every 30 minutes — super convenient! The onsen grounds are quite large, with several terraces (even a smoking terrace with a view — that's a rarity), plus ashiyu, ganbanyoku, a restaurant, and on some days/hours a buffet. Weekday entry is 1200 yen, which includes two towels and pajamas; ganbanyoku is an extra 550 yen. About the onsen itself: there are lots of different baths. The rotenburo area is large. In the rotenburo there's a bath with relatively cool, slightly carbonated water, and all the rest are hot with kakenagashi noted on the signs. There's a lovely shallow bath where you can just lie down with a view of some oaks — beautiful. Indoor highlights: a "silk" bath with white water, plus sauna. I'll attach photos from the site and the water composition below.