Keelung Islet is the dark volcanic hump you see offshore from the harbour and nearby headlands. It is small, steep, and almost without flat ground, but managed landings give visitors access to boardwalks, pavilions, cliff views, and a lighthouse walk when the sea and quota system allow boats to run.
The trip is more exposed than it looks from town. There is little shade, the stair climbing is real, and sailings depend on weather, but that is also the appeal: in a few hours you trade Keelung's port streets for wind, sea birds, lilies in season, and open views back to Taiwan's north coast. Book through a licensed operator rather than simply turning up at the pier.