Get this book!
http://www.hawaiirevealed.com/books-...sland-revealed
It has been a wealth of knowledge for both of our trips to the big island. It is how we found beach 69 mentioned above, which was fantastic, and far less people than Hapuna.
If you dive, and are around the Kohala coast, check out Kohala divers. Robyn is by far the most enthusiastic dive master I have ever met. She is also a Yoga instructor,
. She was so damn friendly and helpful, my wife couldn't even resent how easy on the eyes she was. The whole crew was great. You may not see as much 'big' stuff in the area they dive, but they are great at pointing out some of the smaller stuff in great detail.
Do the manta ray dive / snorkel. Amazing.
Helicopter tour over the volcano is probably the only way you are going to get close enough to see the lava flows. Definitely worth, every penny. Blue Hawaiian is the tour company we used. They are close to Waikoloa, have the newest helicopter, and the most knowledgeable pilots. We did the Volcano / Waterfall tour. Take a video camera, as the video quality of the DVD you can buy is not great. I am glad we still bought the DVD because it has the pilot's commentary in it, and I just spliced my video in where I had good shots that I could sync in. Take a morning flight, the lava contrasts in the dusk better.
If you have not done a Luau, I guess going to one is fun. The Marriot had a decent one.
If you want to see turtles, go to Puako. It is a tiny black sand beach, and there are almost always there. You can snorkel out there (or dive) and there is a 'turtle cleaning station' there, where the fish come in a clean off the turtles. Kinda neat. At the end of the road there, there is another public beach campground area with a nice lava rock bowl to step into to start your swim. Get beach shoes/aqua shoes. Fins with booties is also the way to go, or your feet will get beat up.
The place of refuge is neat to see, close by is the Captain Cook monument, another fantastic place to snorkel. Very mature coral structures. You can take a tour of the Captain Cook monument and learn the story of his demise.
We absolutely love this island. We have been there twice, and have not been to any other of the islands. I would move there in a heart beat. I am jealous, I am sure you will have a great time.
I am missing a lot... really, get the book.