I like the classy and traditional look of six, but after reading some of the posts, I guess number four seems to fit better, although it's outdoors, when it should be indoors.
I second this.
I wonder if it's possible to "fictionalize" a real location into a BG suitable for the VN. Hotels and such usually advertise their facilities with photographs and 温泉 (onsen) and 温泉宿 (hot-spring hotel) sound like good keywords for starters.
IINM, there is another bath scene during the Fanservice arc (Fuurude's house). Maybe one of the rejected BGs could go there.