I will second the suggestion to go snorkling at Hanauma Bay. My wife and I did this and still remember it as one of our favorite memories.
I would check out hotel reviews on Trip Advisor before booking a hotel, unless you already have a good recommendation. We did this before booking, and noticed so many medium-priced hotels that show up on an Orbitz / Expedia search had very bad reviews on Trip Advisor. I got the impression that there are a lot of hotels that are run-down on the inside (but not the outside) that cater to all of the tourists. I believe we stayed at the Outrigger Waikiki. It was clean, nice, and reasonably priced and located within walking distance (a mile or less) of Waikiki, the beach, etc.
Many people seem to feel that Maui is better than Oahu, but we have been to both and much preferred Oahu. It is kind of touristy, but that is part of the fun. There is plenty of nice restaurants, nite life, interesting street performers, etc. It didn't feel touristy in the sense of "lets sucker all of the visitors out of their cash with gimicky, cheap ploys".
We went to Nick's Fish Market for dinner one nite. I would avoid that place. e.g., They charge for every soda refill, food was bad and over-priced, wait staff seemed snotty, etc. We did eat at Tony Roma's Ribs and loved it. I know it is not "Hawaii'n" (i.e., sea food), but still was a great meal.
You must visit the Polyneasian Cultural Center for a day too. That place was funner than I had expected.
Climb Diamond Head if you can. There is also a hike you can take to visit a Waterfall in a park. I don't remember the name, but it was kind of neat.
If you rent a car - get a convertible. The weather is perfect for one. (Just remember, if you aren't use to one, check if the top is going to scrape against any overhead roof, branches, etc. before raising or lowering it.)
We didn't even get a chance to see Pearl Harbor before running out of time.
