I lived on Oahu from Nov '94 to Dec '00. Graduated from UH Manoa. Left for graduate school and haven’t been back (unfortunately), so any specifics I mention may have changed. Some of this stuff only counts if you consider “going to the beach” or “shopping” to be an activity. Also, I’d pick up a Honolulu Weekly (they’re free) to see what events are happening while you’re in town.
If you’re going to be on Maui, I recommend doing a Haleakala bike ride. You take a van to the top of Haleakala before sunrise (it’s cold!), watch the sun come up over the clouds, then ride a bike all the way down the mountain, taking off layers of clothing as you go. Takes about 5 hours total, with a stop along the way for lunch. I figured it was gonna be cheesy and dumb, but came away really glad I did it.
On Oahu, which I’m assuming is where you’ll be, try to get up to Haleiwa. It’s winter, which means big waves! Just don’t get too close to the surf; every winter there are some tourists that get slammed. Of course, if there are big waves, it may not be worth fighting traffic…
If you’re into it, go to club Rock-za. It’s a strip club near the Hard Rock Cafe, across the Ala Wai canal just outside of Waikiki. Plenty of titty-bars on Oahu, that’s just the one I recall the name of.
On the more mundane side, stroll up Kapahulu avenue, visiting Ono’s or Garlic Ahi restaurants. There’s also this Aloha shirt place (I forget the name) that I used to love to browse in; they have all kinds of odd stuff from racks of zippos to cartoon memoribilia to all sorts of clothing. Stop by the zoo and/or Kapiolani park while you’re in the area.
Go to the beach at the Mandarin Hotel out in Kahala. You can park at the public beach next to it (sorry, I forget the name) and just walk over to the left. (This was my favorite beach away from the tourists.) They used to have dolphin shows twice a day; don’t know if they do that anymore. Great windsurfing spot also, as is Diamond Head beach.
Go to Hanauma Bay and pet the fish.
For a cheap, good dinner, stop by Auntie Pasta’s. For some of the best, cheap sushi you can get, stop by Haile’s across from Ward Center. The Dixie Grill was generally a decent eatery also. Ooo - there’s two restaurants my wife and I used to go to all the time for their rice pudding. One was called Scooze’s, the other one was at the other end of the shopping center. Across from the Border’s books and next to the Barnes & Noble, if you want to pick up some reading material.
Hike Diamond Head. They instituted a charge not too long ago, but I think it’s cheap, on the order of a couple bucks. If Manoa valley isn’t too rainy, hike to Manoa falls. Drive up and around Tantalus (it forms a loop from the Punahou area to the Makiki Heights area. There’s a park somewhere along there; drive up the hill, park, and walk out to the scenic viewpoint.
If you’re there on a Sunday, have brunch at the Pacific Reef hotel. $20 when I was there, all you can eat, with a two story fish tank in the middle of the restaurant. Hopefully you get to see them feed the fish.
Go to Duke’s for dinner. I really liked their bread and salad bar, and some grilled ahi with firecracker seasoning was awesome. Go to Dave’s Hawaiian Ice Cream for dessert and try the lychee sherbert.
Most of all, enjoy kicking back. ::gives shaka::