Sounds rough
I haven't been to Oahu except on transfers. Been to Big Island, Kauai and Maui (several times). It's all Hawaii and yet each island has its own character still. Kauai pretty laid back. Maui a nice blend of everything, and Big Island I thought was kinda neat because it had it all (snowboarding and surfing on the same day? Score). When we went though, the volcano stopped. In continuous flow since 1983 but stopped the week (the WEEK, not a word of exagerration) before we got there. They had all these interpretive signs describing what a keen geological event was happening under ground .. and while interesting, we couldn't help but feel "yeah, but what this means is .. there's no glowing lava to see. Boo!".
I've heard it's since started up again. I have to go back I guess.
Kona coast has some really nice diving for coral formations. Lots of large mature SPS reef formations (although Molokini is pretty close - pretty awesome stuff).