A short summary: a unique beach worth visiting if you have the time.
If you have rented a car for a few days, one place to drive to is Punalu’u Beach Park, also known as the Black Sand Beach. It’s on the south eastern part of the island, so will take a couple of hours to get to if you are staying near Kona. However, it’s a great stop to add to your itinerary if you happen to be spending a day near Hilo. Instead of taking a direct route back from Hilo to Kona, you can take the scenic route through the south of the island. You’ll get to see beautiful and unique black sand beaches. There are even palm trees growing in the black sand, and a pond behind a line of trees. This all makes for beautiful scenery.
The black sand is made of basalt, created by lava flowing into the ocean which explodes as it reaches the ocean and cools. This is a result of volcanic activity in the Hawaii Volcanoes National Park.
The beach is definitely too rocky to go swimming in. And the waves are quite strong. Definitely more of a scenic beach rather than one for sunbathing or swimming.
The beach is frequented by endangered Hawksbill and Green turtles, but we did not see any the day we went. We did, however, see several nenes. The nene is a species of Hawaiian goose endemic to the island.
The southern route between Kona and Hilo is approximately 3 hours long instead of the 1.5 hours that the direct road takes. However, there are definitely some stops along the way that make the extra time worthwhile, including the black sand beach.