How about swimming in naturally hot springs – onsens – and then swap to the refreshing water in a waterfall all surrounded by dense subtropical forest? Not only onsens are naturally warm but also naturally rich in different minerals, which is often claimed to have healing and vitalizing forces. At least it is an incredibly pleasant experience to relax in warm water in natural conditions. But unfortunately the civilization is taking over and many onsens are sort of installed into hotels where you of course don’t really feel that it is all natural, and yet more rules are getting rather prude and by numerous requests many of such onsens now require swimming suits, though Japanese have a long tradition of nude bathing (often even in mixed gender groups) in onsens. But here, at Nanadaru Falls, you can experience onsens in relatively untouched state and in absolutely natural surroundings. Though still you should ask permission and pay to the local hotel where you may leave you clothes and other stuff, but probably you should better keep your swimming trunks until you are in the onsen. There are also cave-like onsens, but the highlight of them all is three open-air onsens with different temperature of water (from mild warm to hot) next to 30-meter high waterfalls! And this is only the first (and the highest) of seven (!) waterfalls on the river. It is a very romantic walkway along this forest river with statues illustrating a love-story of a Nobel Prize winning Japanese writer; you may go up to the last 22-meter high waterfall in the magnificent opening of the forest.
To get there from Tokyo and Yokohama use Odoriko Express (or Odoriko-Go) to Kawazu station; ask in tourist kiosk for bus schedule and other information. Bus to Nanadaru Falls goes through magnificent landscape, so you will enjoy absolutely whole trip!
Moreover, you can combine this also with more traditional naturist activity: going to a beach.It seems there are no official naturist beaches in Japan, but Izu peninsula has a lot of sandy and flat rock beaches with warm water and wet subtropical forest around. They are not too crowded, and in the beginning of September it was easy to find a secluded spot. I can recommend visiting flat rocks at Kawazu. They are indeed convenient to dive in the water, as the entrance to water is steep, and then, to lie on, because they are roundish and pleasantly warm. The ocean water is warm too, though the sea weeds rather resemble those of cold northern seas. The view is magnificent and there is also an impressive cave where you can swim
To find these secluded coves walk from Kawazu station towards the ocean, then to the left (in the direction of the tunnel) and you will pass through small forest and rocks; there from above you will see the rocks where you may stay nude, but first you will get down to huge sandy beach. And then just go a little backwards on the edge of the sea till you find the best spot for you.