Besides the regular snorkel places, I had the best just down from the hotel by the breakwater wall. Use to head there most mornings before swim crowds. All the fish were in eating off the wall. Amazing. There was good snorkling in water I could stand in, at least in that area. Most of the best in the other bays like Hanauma bay, were over my head deep.
Loved the Waikiki Aquarium. We must have visited there a half dozen times. One of the best things about Honolulu is just wandering around downtown. Visiting all the places and haggling in the market square. We also rented a car and just bummed around the north shore. That was fun. The bus service is good in Honolulu, if one knew where they were going.
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Of course the Diamond Head climb is a must. Pearl Harbour memorial is good if you can go when not overcrowded like it was when we tried. We still had a good time seeing the other stuff there besides the actual memorial.
Guess its all preference but I would go back there before some of the other places.