Never crowded
When we moved back to Hawaii, we deliberately took a smaller home just for the privilege of living near this beach. It is a great family beach, and on weekends although the lot might be full and the grass park filled with birthday parties and other large groups, once you get to the beach all it takes is a short walk and you'll feel like you have your own private beach.
Sometimes the water is filled with rolling waves and lots of boogie boarders, from children barely out of diapers (like mine) to 90 year olds (one of our neighbors). Other times it is calm as glass and bluer than you can believe. You will almost always see kite-boards and windsurfers a little offshore and kayaks. However, the beach is so wide and long, it never feels crowded.
If you're in Waikiki, make sure to rent a car for the day, drive over the Pali (stop at the the Lookout to get a sneak preview of the beach) drive through Kailua town and get a bento from Fat Boys (hot dog, garlic chicken, fried spam, teri-beef, mahi-mahi and rice for under $6!!) and head for the beach.