Having visited the island of Oahu twice, I will admit that I am not quite an expert. However, everyone has a different experience and I would like to think that mine may help someone else in conquering that eternal question of "what is there to do here?" These suggestions are from things I did, as well as things that I didn't do and may one day get to do!
1. Stock up on 100% Kona coffee. This stuff is amazing, smooth, and goes great with chocolate. You can find quite a few different kinds at the ABC stores, and it's usually cheaper than the $30 bags you get on the mainland.
2. Go to a luau at the Polynesia Cultural Center. These sound amazing, but the luaus can be a bit pricy.
3. Shop at Ala Moana Shopping Center, the world's largest outdoor mall. You may need a couple days to do this if you want to visit every store.
4. Go on a kayak tour of the island. I saw some ads for at least one kayak tour company that would take you into little inlets and lead you over to some sea turtles.
5. I may get a bit of crap for mentioning this one, but you need to try Gyu-Kaku Japanese (but really Korean) BBQ. You get your own little fire pit to cook your meat on, and it's all very affordable. Be ready to wait in line! While we waited, Meng pointed out that, of a group of drunk men getting off a party bus in front of the restaurant, was Jason Acuna. Not sure if it was actually him.
6. Most obvious thing to do: Sunbathe! Lay out on the beach and work on that melanin! Try to keep the pigeons off your towel, as I learned the hard way that they do not know where their restroom is located.
7. Browse the shops along Kalakaua Avenue. You can find anything from a ukulele to designer shoes.
8. If you don't feel like Japanese/Korean food, head to Chinatown for some authentic dim sum. I have been informed by my favorite Chinese girl that Chinatown has some of the best dim sum ever eaten.
9. Tour the rainforest. Lyon Arboretum is approximately 200 acres of greenery, and is a relatively cheap spot to visit.
If you've been to Oahu, what have you done?