Kozushima is a small island approximately 95 miles south of Tokyo, which is accessable only by boat or plane. It has a population of 2,100 residents.
It has many hiking courses around the volcano, Tenjo san, which dominates the island and also a great place for diving. It's white sandy beaches make it an excellent place to swim in summer, since it receives considerably less visitors than the other islands in the chain. Winter visits are discouraged, as you are likely to get stranded on the island due to bad weather.