This is a narrow strip of beach east of Waikiki, tucked under the cliffs of Diamond Head Crater, right by the Diamond Head Lighthouse. It's kind of an out-of-the-way beach, where tourists go for the scenic lookouts, but inadvertently miss the beach down below. If you want to check out a less crowded beach and a good view, stop by Diamond Head Beach Park.
It's pretty close to Waikiki, you could drive or walk, though it is a bit of a walk. Just follow Kalakaua Ave towards Diamond Head and stay on it as it turns into Diamond Head Road. Since the beach doesn't have many amenities you'll have to carry everything, so it's probably better to drive.
You'll see surfers tote their surfboards as they walk up and down the path. Make sure you don't leave any valuables in your car.
Luckily, the path is paved with a guard rail, so footing is easy. You might see the occasional mongoose running through the grass.
And finally, the beach itself. At the bottom of the trail is a shower and water fountain, but no bathrooms and no lifeguards. There are no food stands, so you need to bring your own water and snacks if you're going to lay out for awhile. The beach itself is narrow, the bottom is rocky in places and that means there's a decent reef break further off shore.
The beach extends further east to the sleepy (and expensive) community of Kahala.
Walking further down the beach, it gets rockier. There isn't much to this beach, it's mostly a spot for surfers that want smaller crowds and consistent waves over a reef break. The paddle out and back in isn't too rough. Just remember to check the conditions before you dive in.
here. There's a decent break called Cromwell's (named after Doris Duke's husband) about 100 meters off the yacht harbor.