Ryugo
Wanted to write a detailed review, but realized there's not much to say — it's a standard solid super sento set: several baths (including rotenburo and carbonated), sauna, dining area, ganbanyoku, rest area, massage... There's a free shuttle bus from Kawasaki Station. So if you want to spend a day lounging in baths and soft chairs without leaving, this place handles that well. Keep in mind though that it's not a natural onsen — the water doesn't come from a hot spring source. One "but" from me personally: I'd been to Ryusenji before and was subconsciously comparing Raku Spa to it. And unfortunately, Raku Spa loses. On paper it's all the same stuff, but Ryusenji is better at creating atmosphere, choosing colors and textures for each zone. So if I had to pick between these two super sentos, I'd choose Ryusenji.