Me and my wife went there 2 years ago for our honeymoon... It was the best trip I have ever been on.
I recommend going to Duke's in Waikiki if you can... Absolutely amazing food with a great environment. It's right on the beach so most seats have a good view. There was even a band that came around and sang for each table.
I would also recommend going snorkeling and/or scuba diving. We went to Hanauma Bay which is a nature preserve that you can snorkel at. It was an awesome experience. Tons of different fish and wildlife. We swam around with a sea turtle for like 20 minutes. If you go, remember the sun screen.... Our backs and calves were burnt to a crisp from being on our stomachs in the water.
Rent a car if you can and just drive around. It only takes like an hour to drive from one side of Oahu to the other and the views are absolutely breathtaking.
I also would recommend relaxing as much as you can. Hawaii can be a once-in-a-lifetime type trip so I think it can be easy to want to run around and do a bunch of stuff. We just relaxed at the beach and the pool for the first couple days and I still don't feel like that was enough.
Enjoy man... Like I said, it was the best trip I ever had. Weather was fantastic. It rained enough to cover the ground one time and that was only for like 10 minutes. Hope you have a good time.