This trail in Keelung's Zhongshan District isn't the prettiest walk I've ever done, but it certainly packs in a lot of interesting spots into its short length. There are two lighthouses, port views, a fort, and a subterranean temple.
KEELUNG CAVE TEMPLE
TAIPEI SKYLINE TRAIL SECTION 4 (Short Version)
The highlight of this section of the Taipei Skyline Trail is unquestionably the exceedingly cool temple cave close to Mount Hongdan. That aside, it involves some pleasant wandering over less-travelled paths, and passes plenty of historic spots.
Taiwan Trails and Tales 