The real high point is Pu'u Konahuanui, see my page about it. It is a steep and beautiful trail, but not too terribly hard, and usually pretty clear (minimal bushwacking by Ko'olau standards). You access it from the tantalus area, it is the peak to the right of the pali highway if you are driving towards the windward side from town. http://www.summitpost.org/pu-u-konahuanui/155313
I've spent extensive time in the Ko'olau's coming from both the town (Honolulu) area (Manoa Falls, Jackass Ginger, etc.) and from the Laie and Kahuku areas up North. I've summitted above Laie falls, farther North above Kahuku, above Sunset, at Kahana Bay, above crouching Lion, and also near Tantalus and the pali cliffs closer to town. There are some great mountain races including the HURT 'Top of Tantalus' (which I won in 2007) and the HURT 100 miler (which I dropped out of due to stomach flu). The area is gorgeous and the trails are pretty incredible. That said, I never saw a real good detail of how to get to the actual high point of the Ko'olau's while in Hawaii and what exactly it would take to get there---my guess then was a LOT of bushwacking! When I lived there, NOBODY on the North Shore seemed to know anything about the high point or how to get there.