Located on Oahu, Hanauma Bay is a gorgeous and popular snorkeling destination. Hanauma Bay is easily accessible from Honolulu by car. Just take H1 East, which eventually turns into 72. After you pass Hawaii Kai, you will start climbing uphill. Keep an eye out for the Hanauma Bay sign. Parking is cheap. Just $1 per vehicle. The Hanauma Bay Natural Preserve is open daily (except Tuesdays) from 6:00 AM to 6:00 PM. Come early since the parking lot can fill up. For out-of-state visitors, admission is $5.00 per person 13 and older. After you have purchased your tickets, you have to view an educational video before you may head down to the bay.
If you don’t have your own snorkeling gear, you can rent equipment from the concession booth. One set of snorkeling gear, which includes a mask, snorkel and pair of fins runs $9.00 per person (I believe) for the entire day. They also offer a mask with prescription lenses if needed. If you know how to swim, snorkeling should be second nature. Just head out to the reefs and be amazed at all the fishes out there. Just remember to snorkel with a buddy for safety reasons. The Bay is quite shallow and you should be able to stand up as needed, though try not to stand on the reef to avoid damaging it.
P.S. If you must go to Costco during your vacation, stop by on your way back to Honolulu from Hanauma Bay. There is a Costco warehouse located in Hawaii Kai.