Some of the best hiking trails on Oahu are closed and/or are in disrepair (i.e. Castle, Waikane, Haiku Stairs) but the K2 of Oahu is accessable from Kahana Valley. Mount Ohulehule is "the craggy peak which dominates the windward coast from Kahalu'u to Punalu'u. It stands alone, being only loosely connected to the Ko'olau summit ridge. Radiating from its slopes are 4 undeveloped valleys, Kahana, Ka'a'awa, Hakipu'u, and Waikane. Ohulehule is a classic mountain, beautiful but dangerous."* The climb to its summit from the northwest ridge is one of the most hazardous on Oahu. If you need a guide let me know. It would be my pleasure to take you to the top !!!
* Ball Jr., Stuart M. THE HIKER'S GUIDE TO O'AHU. Honolulu: University Of Hawaii Press, 1993.
I'm sure that everyone will agree that Na Pali coast hike is beautiful--and will certainly give you "memories to last a lifetime!" If you have never hiked Hawaii before, do give it a go!
Um, er, I'm from the Big Island, and believe me, I *love* the volcano area. But, I am suspicious of the 40 mile bike ride on a volcano. It may turn out to be a 40 mile bike ride through lava fields. If you enjoy seeing a desert landscape (I do!), then the bike ride may not be a bad idea. But if the ads show a volcano erupting with 1,000 foot fountains in the background, don't believe it--*probably* won't happen.
See Neal Oribio's write up today on Makiki-Tantalus. That would be a beautiful half-day hike. Since it is close to Waikiki, you won't be wasting time in traffic. And, if you are a speedster hiker, you can take Pauoa Flats trail to Konahuanui, the highest peak in the Koolaus.
Yes, I would recommend leaving Oahu. Oahu has many many beautiful places to offer, but by going to the outer islands, you'll discover that they all have their own unique charm.
Hope you have fun on your trip!