Hapuna Beach is a half-mile strip of white sand about 10 miles north of Kailua-Kona. It's been consistently voted the best beach in the United States, and from what I've seen, I'm going to have to agree. This beach is just great: rarely crowded, nice white sand, calm surf, sunny most of the time. The only problem is the sand gets pretty hot around mid-day, so make sure to wear your sandals.
Beach 69 isn't named for the reason you're thinking. Get your mind out of the gutter. It was named because you parked your car near Utility Pole #69. Used to, at least, until they renumbered the poles. Now you park near number 71, but Beach 71 doesn't have the same ring. Anyways, this beach is nice and secluded, so there generally aren't many people there. The trees come right down to the water, so if you're looking to walk along the beach, you'll have to scramble over some branches along the way. And be warned, you might run across a nude sunbather. We did.