Some beaches, like Kekaha Kai (and beyond), are reachable only by a horrible dirt road. Others are not really beaches at all, like Kapa'a Beach Park, or Puako Beach, away from the hotels. But they are still nice and different. We discovered that snorkeling can be great wherever (Puako, Kapa'a--manta ray sighting!); of course, coral and rock have to be present.
Kekaha Kai (and the other beach close by it) is a gorgeous, nearly empty sand beach. On the Hilo side, we were just at Kolekole Beach Park, where you cannot swim or sunbathe, but you can watch the crazy surfers and sit at tables or on the grass. I just saw on TV that Kolekole at that spot had been devastated by a tsunami many years ago.