Close to Port Allen and Hanapepe is a beach called Glass Beach and near that you find a stretch of coast where people used to dump their trash and you can see what the ocean has done to it. On the lava there are old engine blocks, car frames and so on. The rust has taken an amazing color in some places.
Further down the same part of the coast you’ll find something called the Swiss Cheese Shoreline. Here there are giant holes in the lava where water floods through. One of the holes is called Keyhole Cave (unfortunately I don’t have a good picture of this).