If your have a 4x4 or up for a walk.. go to south point.. There you'll find green sand beach. Most of the locals knows it, so might want to ask around on the best way to get there. But its worth the trip.
Also, in Kau' between Hilo and Kona, theres a black sand beach. It a nice place to swim. If your lucky you'll find sea turtles and swim with them. They also have a small man made (using rocks) fresh water pool. Fresh spring water comes out of the ground and fill the pool, then overflows into the beach.
If you have time the drive around (almost around) the island is well worth it. You get the see a little of everything and stop those little restaurants.
Funny thing is saw was a little stand on the side of the road with a bunch of fresh/ripe papayas and mangos. No attendantin site, only a box that says "please pay here". We thought that was funny..