As promised, I am back and will now give you some tips. I was only there for 5 nights and had a child with me, so I didn't do some of the cooler stuff like diving.
1) Go to the lava tube at Volcano and do it so that you arrive just before sundown. This will keep you from roasting on the hike out and allow you to see the glow of lava when the sun sets. Bring a flashlight to get back. There are lights to help you navigate back, so don't worry about getting lost.
2) The little beach right next to the Outrigger Hotel at Keahou has some excellent snorkeling. It's crowded, but worth it. I understand Cook's Monument is the best snorkeling and diving, but you kayak to get there. Couldn't do it with the little one. Get a mask and snorkel that don't leak.
3) Green beach is cool, but freaking windy and you really do need 4wd to get there. Get the extra insurance if you plan to take your 4wd rental on some of the rough trails. You could park and mountain bike there or even hike the 2 1/2 miles. It's very windy and dusty on the way there.
4) Get a guide book and the best detailed map you can possibly find before going. They don't want you to be able to find your way around. Nothing is marked and maps suck. It's hard to even tell what road you are on. You navigate primarily by mileage markers on the highway.
5) Drive up Mauna Kea and slide in the snow. Not worth skiing when I was there but fun for a slide on the butt.
Have a mai tai. Enjoy your trip.