Green, mountainous and mostly unspoiled, Dominica is an intriguing alternative to the more crowded Caribbean islands. Sparsely populated and 30 miles long by 15 miles wide, it is volcanic and does not have the white-sand beaches found elsewhere in the region. But that has spared it the heedless development that has marred some of its neighbors. Almost one-third of the island has been set aside for nature reserves.  It features impressive peaks, some almost 5,000 feet tall; spectacular waterfalls; and dense tropical rainforests that allow more than 1,200 species of plants and 175 species of birds to flourish. Because of the challenging terrain, roads mostly follow the coast. With three national parks, more than 300 miles of hiking trails and world-class diving, the island offers innumerable activities.