Kailua Beach is always named one of the best beaches in the world. It has the most deliciously soft sand, gentle surf, and maybe a turtle ( honu) to swim with.
Kailua Beach is actually many beaches. For a fascinating study on ancient beach names, take a look at the historical Hawaiian names in John R.K. Clark's book “The Beaches of Oahu.” In modern times the beach names are three. Oneawa ( AKA Castles), lies to the north near the house the President favors for holidays. Kalama sits in the middle of the crescent named to honor Queen Kalama the wife of King Kamehameha III, former owner of the entire Kailua district. Kailua Beach Park is thirty acres with parking, picnic facilities, lifeguard and restrooms. Kayak rentals are close by and you may pull the boats across the street, stopping for a custom made sandwich at Kalapawai Market, and paddle out to one of the Mokulua islands.
If you want a quieter experience albeit with no facilities or life guard, try Kalama Beach via any marked public right-of-way along Kalaheo Avenue beginning in 300 block and proceeding north. Pay attention to no-parking signs; find parking down a side street off Kalaheo Avenue TOWARDS the mountains. Leave nothing of value in your car, take chairs, towels, sunscreen, hats, a cooler of water and perhaps lunch. If winds pick up, pack up and head to docile Waikiki.