Just returned from five fantastic nights in Honolulu. Such a beautiful island.
Scuba diving. We used Dive Oahu, whose office is located right at the Kehalo Basin near the Ala Moana beach. A two tank dive is about $130, and you can dive even if you're not certified. WARNING: We had originally booked with Reeftrekkers, but they cancelled on us at the last minute (we were already at the pier when they called to leave us the message) citing "ocean conditions", even though the ocean was calm (Dive Oahu took us out the same day and the ocean was fine). I think Reeftrekkers just didn't have enough people signed up. Stuck with Dive Oahu and you'll be fine.
North Shore. We watched the surfers at Pipeline and then went swimming at Waimea Bay. The waves were huge! I spent about an hour and a half in the water riding the waves(a highlight of the trip); be careful, because the ocean here is rough and not appropriate unless you're a good swimmer (an obese gentleman had his bathing suit ripped from his body while we were there!). On the way to the North Shore, we stopped at the Dole plantation for some wonderfully kitchy pineapple-themed fun.
Waikiki. We loved it and loathed it. It's trashy and classy all at once. It was nice getting mai tais at the beachside bar the the Royal Hawaiian at sunset. Avoid "Cheeseburgers in Paradise", a lame little place on the main strip. Overpriced.
Happy Hour. Martinis at Indigo restaurant, a fashionable little place in Chinatown, are $3.50 before 6pm (and you can even specify the brand of liquor!) stay for dinner; the fish specials were unbelievably good (mine was Moonfish in sizzling Scallion Oil...mmm...)
Cocohead. A fifteen minute drive from Honolulu gets you to this small mountain. A twenty-to-thirty minute stair climb brings you to a spectacular 180-degree vista of Honolulu, Diamond Head, the Atlantic, and the mountains. A tough climb but very satisfying.
Amuse Wine Bar. In the Design Center at Kapiolani and Pi'ikoi. A trendy place to sip one-ounce portions of wines for $1-$3. Grab a glass, and try ten different wines in one sitting! An even better deal is at happy hour on Sunday and Thursday, when the wines are all half-price! (Read: $2.50 for a five-ounce pour!)
Gay Nightlife. Not great, but servicable. We danced at Fusion to some fun pop remixes; beforehand we grabbed beers at Angles. Hula's has a good happy hour special, but avoid going there at night when they have a cover; it's like paying a cover to go to a pub.