Kailua
Short but steep 3.4km round trip to twin WWII pillboxes on the ridge above Lanikai Beach. The view of the Mokulua islands (two offshore islets) and turquoise water below is one of Oahu's best. Go at sunrise for the best light and to beat the heat and crowds. Combines perfectly with a Lanikai or Kailua beach morning.
No booking required. Free. Trailhead on Kaelepulu Dr in the Lanikai neighborhood. Very limited parking — park in Kailua and walk or rideshare to the trailhead.
Go at sunrise — the ridge is fully exposed and becomes very hot by 9am. Slippery when wet.