Guishan Island, or Turtle Island, is the volcanic shape you see offshore from Toucheng. It is part of the Northeast and Yilan Coast National Scenic Area and is managed with visitor controls, so trips depend on season, permits, sea conditions, and operators.
The island itself has fumaroles, cliffs, and a strong silhouette, but many visitors experience it from the water on whale- and dolphin-watching boats. Check the rules before building a whole day around landing there.