One of the main attractions in Hawaii is beaches.  I'll start a running list of beaches starting from Ala Moana Beach going east (or counter-clockwise) around the island, but they're a lot of beaches to cover. Whether you spend a few afternoons or your whole time on the beach, it's up to you.  I'll just post it.  Just bring plenty of sunscreen, water, and towels.  Don't forget to check the local beach conditions, depending on the beach and time of year there could be strong rip currents or unseen beach breaks.  As long as you're aware of the local conditions, you should be alright.    

Ala Moana Beach Park Officially called Aina Moana Beach, but nobody calls it that.  Flat, sandy beach near Ala Moana Shopping Center.
Waikiki Beach If you're staying in Waikiki, the beach is almost right outside your doorstep.
Diamond Head Beach Park Semi-secluded beach located right under Diamond Head.
Hanauma Bay Beach Park/Natural Preserve Best place to snorkel with sea turtles and fishes.  Located on the east end of the island.  
Halona Cove Beach and Sandy Beach Two beaches located near the Halona Blowhole.  Halona Cove Beach is a small beach without a lot of amenities.  Sandy Beach has a rough shore break and is for advanced boogie borders and body surfers.  
Makapuu Beach Park Near Makapuu Lighthouse and Sea Life Park.  Sandy beach with rocks and coral, great for boogie boarding, but shore break and riptides making it hazardous for swimming in winter months.
Waimanalo Beach Park Eu-jeania's favorite beach.  This beach is not crowded, has free parking, restrooms, showers, lifeguards, sandy bottom, and low surf.  
Waimanalo Bay One of the state parks, this place has the same facilities as Waimanalo Beach but is bigger.
Bellows Field Beach Park Bellows Field is open to the public on the weekends and holidays.  Good surfing, but can't surf between the two lifeguard towers and watch out for the occasional jellyfish.    
Kailua Beach Park and Lanikai Beach Listed as one of the top beaches in the States, this is a good beach for swimming, windsurfing, and kite-surfing.  Most people have heard of Kailua Beach Park, but it's actually two beaches with Lanikai Beach being a "hidden gem".  
Laie Beach Park Also known as "Pounders Beach" for the fierce waves pounding the beach.  Good spot for beginner or intermediate surfers.  There's another section of the beach protected by a reef called "Bathtub Beach" because it's calm waters.  That part is good for swimming.  Beach is nearby BYU and Polynesian Cultural Center.  
Hukilau Beach and Hukilau Cafe This beach is a little off the beaten path.  No facilities here, so you're on your own.  High winds make this a great place for kite surfing in the winter. 
Sunset Beach Another beach best known for big waves and great surfing, part of the Vans Triple Crown of Surfing.  Waves are up in the winter months, summer months are good for swimming.  
Banzai Pipeline Also called Ehukai Beach, this is one of the premier surf spots and part of the Vans Triple Crown of Surfing.  In winter the waves are fierce and best left to the professionals.  In summer months, surf is smaller and good for swimming.  
Waimea Bay Beach Park Best known for big wave surfing in the winter, part of the Vans Triple Crown of Surfing competition.  Sandy beach with a small parking lot, not recommended for swimming in the winter months due to high surf and undertow.  
Shark's Cove Rocky beach with reef that provides a home for sea animals, like octopus and sharks.  Good place to go snorkeling and diving.  In winter months the high surf obscures the reef and makes entry hazardous.  
Ko'Olina Beach Check out this "hidden beach" at the West end of O'ahu.  It's four man-made lagoons on the Ko'Olina Resort.  The resort is private, but the beaches and parking are public.  The lagoons shelter the beach(es), so the waters are calm for kids and beginning swimmers and it's less crowded than Waikiki.