If you want to to something a little different:
Hawaii Shark Encounters will take you out from Haleiwa on the North Shore to a spot and put you in a cage with 4 to 6-foot sharks swimming all about. If the surf isn't bad you shouldn't get seasick, but if you are adventurous enough, it's still worth a little chunkage over the side. My wife and I would do it again in a heartbeat.
There's a tour that circles the island in a Hummer that hits all the places they film TV and movies like Lost, Hawaii 5-0, etc. Lost was still big at the time, and I was a huge fan when we took this tour. It goes all over the island.
Keep an eye on the surf forecast for the North Shore. If they're breaking, you must go watch Pipeline or Waimea especially. If you just want to experience your own awesome beach scene for yourself, you won't to better that Kailua Beach. I could spend the rest of my life there.
And Hanauma is the only place to snorkel. Place is awesome. And can get crowded.
Have a blast.