I was in Oahu over the holidays visiting the future in-laws. Spent a few days on the Big Island. I've only done those two, see Oahu post before, Maui & Kuai next time. My fiancee is from Oahu, they like Big Island the best. Fire and ice. More for the adventurous, not as much for the R&R beach goers. But the beaches are great too if you want both.
Volcano National Park was awesome. We hiked to the lava at night. Much more daunting hike than expected. Miles over undulating lava rock in the dark not knowing if we'd fall in. Pro-tip: start 3-4 hrs before sunset. Not for kids, not for non-adventurous, not for the faint of heart. People were hobbling back with sprained ankles. The payoff was worth it. A few meters from flowing lava, unreal. The caldera at night was cool too.
The giant manta ray night snorkel was unreal too. Not for the faint of heart either. Cold water temps while sustaining a flat arm outreach position for 45 mins. Adrenaline keeps you going. Older kids only. They come up to within an inch of your body. Don't touch!
Do a snorkel tour out of Kona. Fine, for kids. Maybe 10+? Captain Cook, Two Step, and remote southern location. They'll show you all the coastal lava rock formations. Dolphins too. Pro tip: At Captain Cook, go out among the rocks at the point, that was best. You're almost pushing tropical fish out of the way. Make sure you find a Honu.
We stumbled upon Merriman's as mentioned by
@storrsroars. Delightful. Don't leave without trying the bread pudding.
Stay in VNP, stay in Kona, rent a 4x4 (For Green sand beach and Mauna Kea summit). We had an amazing Airbnb for $100/night in Kona.