I used the Hawaii Revealed
Hawaii Revealed | The Ultimate Travel and Vacation Guidebooks tourbook (there's a different one for each of the islands) on my trip, and must admit that almost everything I did was pretty accurately described in the book.
You must have a car. The island is about the size of Connecticut. I don't think you'll be happy stuck in one place the entire time. There's so much to see.
I regret not having done the Waipio Valley as someone advised above. But then again, the vacation's only so many days long.
It's the only Hawaiian Island with no beaches. Well, not literally. But your image of Hawaii is probably an island surrounded by Waikiki-type white sand beaches (and some black and pink and green). The Big Island has those, but it's a lot of black lava rock.