Hi there. I've lived on O'ahu for about 9 years, for reefers one of the best spots is Hanauma Bay (hands down...the best, for the money and accessibility). I've snorkeled there many times, and this last time, there was a 2 foot naso tang about 5 inches from my face. Fishes are very docile and all I believe are resident fishes. Gobies, urchins, etc...as well.
Drive on Kalanianaole Hwy heading towards Hawaii Kai. THe drive itself is worth it. One of the best on the islands. The roads will curve as you see the ocean from the hillside.
If you have snorkel gear, bring them as luggage, or you can buy cheap ones at one of the ABC stores...or rent them at Hanauma.
If you want to see sharks...there are lots of hammerheads at Kaneohe Bay...by Chinaman's Hat..
Other beaches that are must see are...Haleiwa (surfing town, literally), Waimea Beach, Sunset Beach....Lanikai & Kailua Beaches...Sandys Beach... Don't forget to eat at the shops off the roads. Some are the best on the island.
Best,
Ilham