If you're in the Vík area and still have time after visiting the Reynisfjara black sand beach, you might consider doing the beautiful hike up Hatta mountain. From the top of the mountain you'll have amazing views of the surrounding landscape, including Reynisfjara and the town of Vík.
About the trail
The hike starts at Víkurkirkja, the church of Vík. There is a small parking area around the church. As this is a lesser known hike you shouldn't have any trouble parking. You'll follow a path up to the summit of mount Hessa. Here you'll have a great view of the surrounding area. You'll take the same way back.