Located at the top of a steep, vertiginous road, Shoukichi no Yu is a simple place with unadorned grounds that verge on shabby, but it has a lot to offer, especially for larger groups. You could easily come here with a bunch of friends and feel like the place is all yours -- and the views of the valley are sublime.
Getting here will, no surprise, require a car and a pretty good GPS system. Your closest town is Yokokawa, but it's tiny. That's not a drawback: come here to get away from it all and experience a hot soak amid the pastoral beauty of the Izu peninsula. Beautiful fruit trees, brilliant rice fields, and a soaring hawk high above you make it well worth coming. If you bring your own dinner you can rent the barbecue facilities and cook your own meal after the bath. The down side is that whereas the building has some gorgeous woodwork and almost resembles a temple atop the hill, it's marred by green plastic astroturf and plastic PVC pipes that haven't been at all concealed. The water is hot enough but not broiling (a good beginner bath); in winter it may be too tepid to be worth the trip.