Check out our post in the "Oahu" section about Hanauma Bay.
You don't have to snorkel, of course. You can just hang out on the beach like Yeonhee.
There is a snorkeling gear rental shop on site.
Don't forget to bring snacks, hats, beach towels, sunscreen, water and etc...
7455 Kalaniana 'ole Hwy, Honolulu, HI 96815