If you're a nature lover or an avid hiker, Yakushima (屋久島), designated a Unesco World Heritage Site in 1993, is one of the most rewarding islands in the Southwest Islands. The craggy mountain peaks of the island's interior are home to the world-famous yakusugi (屋久杉; Cryptomeria japonica), ancient cedar trees that are said to have been the inspiration for some of the scenes in Miyazaki Hayao's animation classic Princess Mononoke.
Keep in mind that Yakushima is a place of extremes: the mountains wring every last drop of moisture from the passing clouds and the interior of the island is one of the wettest places in Japan. In the winter the peaks may be covered in snow, while the coast is still relatively balmy. Whatever you do, come prepared and don't set off on a hike without a good map and the proper gear.