The area around the lighthouse at Diamond Head has long been frequented by naked people. It's right in town, just at the far end of Kapiolani Park from Waikiki. There's almost no sand - it's more of a cliff face. And it's NOT a "free" beach - there will be occasional clothed walkers, and there is occasional enforcement if things get a little out of hand. Be discreet - stay in the water, or tuck yourself up into a crevasse in the cliff. That said, you will likely find a handful of other naked folks there. It's a nice walk, anyway. Play it by ear.
Aloha from Hawaii. We have tried to find the nude area near Dimondhead, a few times, with no luck for us. We normally go to Polo Beach, when we get the chance. It's all the way on the North Shore of Oahu, across from Dillingham airfield. The trail is between two polo grounds, and is very easy to miss. When you get there, take the path to the beach and turn right, left side is clothed. Find a nice comfortable spot and enjoy. Too far to the right is mostly a gay area, but they are never a problem. Most of the folks there are friendly and very welcoming. Have not found a resort or anything like that on Oahu. Hope this info was helpful, and hope you have a wonderful time when you get here.
Lilo & Stitch.