The popularity of hiking Rishiri-zan overshadows just how much else there is to do on this island. Local adventure guide Watanabe Toshiya offers backcountry skiing tours (late December to May) on Rishiri-zan and sea-kayaking excursions (June to September) around the island. You'll need a good dose of experience and, for backcountry, your own equipment.
His backcountry tours (about ¥10,000 per person per day) are particularly popular and book out fast, especially for peak powder season in January and February (April, he says, is best for sunny days). This is true backcountry stuff (think no lifts) with plenty of hiking up, followed by steep descents down the volcano.