Peacock Flats is hardly ever fully booked out due to how remote it is and requiring 4wd to get there. It's quite the adventure and can be very wet and muddy. Highly recommend having paperwork, permits, IDs all together in advance because you have to pass through military checkpoints on the west side of Oahu to get through to road to the campground. They ask for everyone's IDs. There are a couple trailheads for hiking and nice views of the north shore and west side along the drive to the campground. The drive in can take 1-1.5 hours or longer depending on the conditions of the road. Come prepared! I also recommend having an air compressor, spare tire and tow equipment if you need to save anyone or be saved yourself lol. Check the conditions beforehand. If it's rainy season and you don't have experience driving in steep muddy areas with large dropoffs and cliffs on the side, then maybe wait till summer when it's drier!
The sites can be quite wet, we hammock camped the first time we came out to stay off the ground. There are pit toilets and picnic tables under little shelter areas. Definitely recommend for the adventure and if you love off roading.