If you go to Oahu you have to go snorkeling at Hanauma Bay! The bay is protected from large waves and the reef is just a few feet from the water's edge. The water here is shallow and crystal clear.
There is a required informational movie that you view before you are allowed to snorkel. It tells you about how the bay was formed and about how to protect the fish and wildlife that you might see.
You can rent snorkel gear of take your own. If you are new to snorkeling they suggest renting a "dry set" for $9 a person. They do not take credit cards so make sure you take enough cash. The nearest ATM is in Hawaii Kai, a few minutes drive from Hanauma Bay. You can also rent a locker for $5.
There are lifeguards on duty and and information area down at the beach. It is a long and steep walk down to the beach but there are shuttles that go up and down for a few dollars. The Bay is closed one day each week to give the wildlife some rest. It is currently Tuesday but double check this before you go!
When the parking lot fills up they do not let anyone else into the park so get there as early as possible. Parking is about a dollar and I think admission was under $10 per person. Around $5 or $7. Take an underwater disposable camera to catch pictures of the coral and fish you'll see!