I've lived here since the summer of last year. I haven't even scratched the surface on things to do. Hikes, beaches, tours, snorkeling, diving, boatrides, fishing. on and on and on.
I lived on Oahu for 3 years, and also didn't scratch the surface! Of course I was busy becoming a mom there too (my son was born there halfway through my husband's tour in the Marines).
We now go back every other year (own at Marriott Ko Olina), and a few years back, I booked the kids and I to do the Atlantis submarine that goes from the Hilton Hawaiian Village. It was really fun!
We used to sometimes stay in Waikiki either before or after Ko Olina, but now we try to just do the condo if we can.
We like to go eat at old places that we used to go to - in Kailua we used to always eat at Willow Tree Korean restaurant (my husband is Korean, born there), so we went back a couple times years later! Fun and such good food.
My daughter loves the flea market (swap meet). She wears her tee shirts to sleep in all the time.
I used to take visitors to Pearl Harbor, but have been so many times that I would bring a book & just sit and relax LOL. Definitely you can't visit Oahu and not go there.
I'm not into shopping so really don't go to Ala Moana mall, but if we do go we eat at Shirokiya!
I agree about ramen places in Waikiki - my husband LOVES that stuff! Also any of the Korean eateries are usually really good - the more hole in the wall the better!
For someone first visiting, I do think it's fun to stay in Waikiki for at least 2 nights, then go over to Ko Olina - you will see the difference. It's so relaxing that often my husband doesn't want to leave once we get there. I take the kids around sightseeing, and he stays and enjoys the resort. I remember when it was just one hotel - we would go over from Kailua for dinner & drinks and it was so peaceful, like going to a neighbor island. It's still so much nicer than Waikiki to me!
If anyone in your party is interested, the Waikiki aquarium is cool. It's small and cute, and my daughter who loves animals liked it! Husband and son, not so much! Can't please everyone!