At the end of the Big Island, on the south-eastern side, you can find a black sand beach, one of the newest on the island, and see huge green sea turtles up close.
The map shows the location, but there are multiple parking lots around, so find a spot during the afternoon shouldn’t be a problem. You’ll likely fit this in with a trip to the Volcano National Park.
Remember to keep a safe distance away from the turtles as these are wild animals. Otherwise, enjoy their company as you view from afar.