Mrs. Outoftown and I have been to the Big Island several times and are going again in January 2011. I think we have absorbed enough aloha spirit to get a feel for the place and am surprised that no one has yet mentioned what locals call Warm Pond, Vacationland or one of the best kids snorkeling spots in the world; Kahaluu Beach Park, just south of Kailua-Kona. We go on the cheap, but I don't think eating at Ruth Chris would be a "must-do" in Hawaii. Personally, Mrs. Outoftown think Roy's at Waikoloa Village rates better since it is Hawaiian fusion fare. I prefer Costco sushi and the surfer food sold out of huts near the beach myself. It beat's Fred's House of Pancakes, anyway.
We have yet to see Mauna Kea and the manta rays, so those will be must do's for us this time.
I do recommend black sand beach, green sand beach, volcano national park (on a day when it's not raining).
-outoftown