Punalu'u's Black Sand Beach is a MUST SEE in Hawaii. There are very few true black sand beaches - and this is one of them. It's easily accessible right from the main highway around the island - just follow the signs when you get between mile markers 55 and 56. Park (free), take off your shoes, and hop out to be awed by the color nature takes here - esp beautiful on a sunny day (though the sand can get HOT). The black of the sand, blue of the sky, and green of the palm trees and lush Hawaii make for a wonderful contrast.
Taking off your shoes for at least a little bit is critical. Why? Because true black sand doesn't FEEL like ordinary sand, and it's hard to explain. A few steps and you'll understand. It's neat and definitely different.
You can swim on this beach, but the water is cold—especially due to some freshwater that comes up from the ground here (you can see it along the water's edge). We opted to wade and take in the beauty. Our boys enjoyed playing in the water and on the rocks. There are plenty of turtles around if you haven't seen any on the BI yet.
Tour buses stop here, as does pretty much every tourist circling the island, so be prepared for crowds. Even so, it's worth it to stop. When the buses aren't there the crowds are quite reasonable, so wait a little bit if they are there when you arrive. They don't stay long. It's a fine place to picnic if you are so inclined, but we didn't. (We wanted to eat at the Southernmost restaurant in the U.S. [see separate journal for Hana Hou Cafe].)
This stop makes a nice stretch of legs if one is going to/from Volcanoes National Park and Kailua-Kona. It's a shorter beach, but nonetheless, the beauty makes it nice for beach walking.
If you want some tourist souvenirs, there are shops here.