While on the Big Island, we found lots of beaches and parks that were labeled and easy to find. But the place we liked the most, because we found it accidentally, didn't even really have a name. When I told some locals about it, they swore we were in Kolekole Beach Park, which we were NOT because we did go there too, and this was a different place. We found a locals-only hangout, and when we arrived (under the enormous highway bridge which looks cool from below) we found people with their car doors open, listening to music from cranked radios, drinking 40s of malt liquor and just watching the tide roll in as the sun went down. Total perfection!
The best I can tell you is to take 19 north from Hilo past Kolokole Beach Park. Take a right on to Old Mamalahoa Highway or Wailea Road (depends on what navigation site you use, but it's the same road and you'll pass a tiny post office while on it), and then start looking for some access road that isn't labeled, but clearly descends way down. Once at the bottom, we had to drive across a concrete bridge with water flowing over it to reach our spot. It may not be the fanciest place around, but it's totally real.