This hike climbs Poamoho Ridge to the Ko'olau Summit. You start off on a wide contour trail through mixed jungle, with occasional peeks at Poamoho Stream below. The upper half of the trail becomes rougher and steeper but not especially difficult. Eventually you reach the summit at a windswept junction with panoramic views over Punalu'u and Kahana Valleys. It's worth turning south along the KST for a half hour or so. The trail offers continuous views and is carved dramatically into the face of the cliffs.
permissiondogs leashedyear-round4WD vehicle
This hike is harder to access than most. You'll need a 4WD vehicle plus a hiking permit from the Department of Land and Natural Resources:
http://dlnr.hawaii.gov/recreation/nah/oahu-trail-access-road-maps/poamoho-permit-application/
The DLNR makes the process pretty simple but you're limited to 5 people per group and hiking is allowed on weekends and holidays only. They recommend applying at least 2 weeks in advance to allow time for processing.