If you're a nature lover then you're spoilt for choice!
There are plenty of things to do on Yakushima. It's an 'outdoor person's' paradise. For such a small island there are an abundance of trails running in all directions across the island. Some of them are well-trodden, but others require some very vigilant navigation. Winter conditions can also pose some difficult challenges for the serious trekker. There are also some easier paths through the forests that can be just as rewarding. Yakushima has a long tradition of pilgrimages up to the mountains to request good harvests and health from the gods. Many of the smaller mountains close to the coast have these old pilgrimage routes. Many hikers on Yakushima are unaware that many of the trails they walk upon have been used as a pilgrimage for centuries, if not longer. The central highland was/is a sacred area for the Yakushima inhabitants and this is an issue that UNESCO largely ignored when they designated the central region of the island as a World Heritage Site of Natural Beauty. They placed the emphasis on Yakushima's natural wealth at the expense of its cultural wealth. When up on the mountains then please respect these places as of spiritual value to the local inhabitants and should you wish to gain a good insight into the area you are traversing then consider a YES Tour as being a way of getting the most out of your visit to the island as the guides turn your experience from a 2D to a 3D impression.
Once on the island you will quickly realise that there is also an abundance of water. The rivers provide great leisure areas and the clarity of the water is quite amazing. The sea also plays its part in providing things to do: scuba diving, snorkeling, swimming and fishing (both shore and boat fishing). YES can arrange a great day out on a chartered fishing boat. Contact YES for details.
Even when it rains there are a good range of museums to keep you occupied. There are some indoor hot springs as well as a few outdoor hot springs by (or in the case of Hirauchi hot spring in the sea!). In heavy rain then the waterfalls are at their best.
Like many areas of natural beauty, Yakushima attracts a high proportion of artisans inspired by their natural surroundings. You can usually visit these artists and watch them at work. Some places even allow you to make your own artifact with the craftsperson by your side.