We spent a good half of our 3rd day in Oahu at Hanauma Bay, the entrance fee is $7.5 for non residents and you will have to watch a 20 minute documentary/video on the preserve (if you sign up at the end of the video you will NOT have to watch it again if you return within a year).
After you descend to the beach you will be able to rent snorkeling gear and (a limited amount) of lockers. We found out later that you can actually purchase the snorkeling gear in Honolulu for the same amount of the rental fee, you call...).
Up to then we did some splurging on tours/rides and expensive dinners and only after Hanauma we realized we were doing it "wrong", enjoy Hawaii like the Hawaiians, for the price of admission and gear rental we had the most fun filled 1/2 day in Hawaii.
Consider getting a waterproof digital camera (we got one specifically for this trip), you will run out of film quickly if you get a single use one.
Go there early (before 10 AM, closer to 8:30 AM).
Pack a lot of sunscreen (the Hawaiian sun is pretty hardcore).
You might want to take it easy the next day, you will be sore after swimming all day long.
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