Although only 20km west of Ishigaki-jima, Iriomote-jima (西表島) could easily qualify as Japan's last frontier. Dense jungles and mangrove swamp blanket more than 90% of the island, and it's fringed by some of the most beautiful coral reefs in Japan. If you're super-lucky, you may spot one of the island's rare yamaneko, a nocturnal and rarely seen wildcat (they're most often seen crossing the road at night, so drive carefully).
Several rivers penetrate far into the lush interior of the island and these can be explored by riverboat or kayak. Add to the mix sun-drenched beaches and spectacular diving and snorkelling, and it's easy to see why Iriomote-jima is one of the best destinations in Japan for nature lovers. That said, if you're not an avid explorer of the great outdoors, you're likely to find the slow decline of Iriomote's tourist infrastructure (what little there is) somewhat disappointing.