Nana no Yu (菜々の湯) is a large bath house within walking distance of Keisei-Okubo station in Narashino, Chiba.
There several baths, such as carbonic acid bath, an electric(?!) bath, a stone barrel bath, an outdoor bath, and two saunas. There’s also a stone spa for an extra fee, which I haven’t tried yet. If you want to double-dip, there’s a lounge area where you can relax.
Don’t forget to bring a towel.