I did the big island, it really is big (especially when compared to Maui). Kona is great, lots of really good snorkelling, esp just south. There is a spot next to a state park (name escapes me at the moment) where we got to see those stereotypical scenes of a turtle diving with a cloud of yellow tangs surrounding it. We thought about the manta ray dive, but it goes quite late, plus we were chicken!
Upcountry there are coffee plantations, nice hikes, etc.. It's quite a drive to volcano national park (2 hours?) so plan on either a long day or book an overnight. If there is a active eruption it's totally worth any effort to see it. Last we were there there was an eruption right on the coast. Once it starts to get dark what looked like just a huge steam plume with some rock/distant the bottom is clearly glowing red lava.
It's just really hard to go wrong wherever in Hawaii you go.