We were there about seventeen years ago. On Oahu, we stopped at the Arizona Memorial. They also have a submarine there that you can tour. The Punchbowl Cemetery was mind numbing to say the least. I went to the top of Diamond Head, however the wife declined to go all the way up, there were some pill boxes from WWII there. Was not to bad a walk at all, other that all of those steps. We also went on a harbor supper cruise, ok. The harbor there was the harbor that was shown on Gilligan's Island, the old tv show. There is also a place there that shows you the areas of island life in the pacific and how they lived there, in my opinion a pass, half way through figured out it was part of the Mormon college as all the presentations sounded the same. We did not spend much time at the beaches as neither of us swim. Total time spent there was three days, figured we had seen it all there in that time period.
On the big island, we spend a week, but is was a business trip, so we spent one day driving around the island, seeing Volcanic Park, the waterfalls etc. The largest ranch in the United States is on the big island, it is called Parker Ranch. On the big island you can go surfing and snow skiing on the same day.