The Brough of Birsay is a small island just off the Orkney main island, accessible only during low tide.
Upon walking the concrete path over to the island, the first thing you encounter are viking ruins from the 8th or 9th century. These ruins are quite extensive and are in great shape. There are no barriers and you are on your own to respect and admire the ruins.
On the far side of the island are high cliffs where you can watch the waves crash into the rocks below. There are several places where you can stand with your feet spanning 100-foot-deep chasms. Beware of the time, however. You can only cross a few hours before and after low tide. If you don't pay attention to the time, you may find yourself stranded.