A day loop from Nagasaki city around the Shimabara Peninsula — the finger of land that separates Ariake Bay from the East China Sea. The route visits the Atomic Bomb Peace Park, climbs to Unzen's steaming hell vents (jigoku) where volcanic fumaroles have been boiling continuously for centuries, descends to Obama Onsen's famous outdoor foot bath on the Ariake Sea shore, and ends at Shimabara Castle before returning to Nagasaki.
Clone it to your planner, customize dates and stops, then ride. All free.