Punalu‘u Black Sand Beach Park, 1.5 mi. s.w. off SR 11 (the signed entrance road is between mileposts 55 and 56), is perhaps one of the most easily accessible of Hawai‘i Island's black sand beaches, formed when hot lava meets cool ocean water and shatters into tiny pieces. Wave action further pulverizes the black volcanic rock into grains of sand. Backed by tall coconut palms, the crystalline black strand is very popular with day trippers traveling between the Kailua-Kona area and Hawai‘i Volcanoes National Park; count on seeing tour buses here.
The shoreline is mostly rocky, and ocean conditions are generally rough (beware of rip currents), but you needn't swim or snorkel far from shore to see the bay's resident green sea turtles. In fact, you'll often find them lazing on the sand, surrounded by groups of camera-happy tourists.
In addition to picnic tables, a campground located off of the beach, restrooms and public showers, there's a bungalow with vendors selling snacks, drinks and souvenirs. If the first parking lot you encounter (the north lot) is full, there are more spaces just down the road in the south lot.
Note: Green and hawksbill sea turtles are threatened species protected by federal and state laws. Harassing or touching them carries heavy fines. A permit is required for camping and picnic pavilion use.