I'd probably do the 9-day of Kaua`i and the Big Island. Ko `Olina is nice enough, but in light of what your posted goals are, not enough to warrent a visit on your itinerary.

The beach at Waikoloa is fair enough, but a few miles north is Hapuna Beach, which is one of the best in the state. Also, don't discount the size of the Big Island; the drive from Waikoloa to the Volcano can easily take over three hours.

My $.02