Polihale is our favorite drive in campsite on Kauai. It's far enough away that it's rarely crowded. On the weekends it's definitely a local spot for camping and surfing. You can drive your 4wd out on the sand. Or a normal vehicle can make it down the end of the long dirt road and you can walk a little ways onto the beach and set up your tent on the sand. There are bathrooms and outdoor beach showers. I don't think it's potable water though so bring your own! :) the large long beach is unbeatable and the towering mountains at the start of the Na Pali coast is spectacular!
Camping on the beach doesn't get any better. Amazing white sand against Na Pali cliffs, we whale watched from our tent (with binoculars) The dirt road can be rough and there are not services once you get out there. We saw some surfers, but didn't swim ourselves; they say to beware of strong off shore currents. The only downside was the chickens did wake us very early. Book early for your permit if you have a specific night in mind.