Castello di Siliqua. This ruined castle lies a top a hill and offers a beautiful panorama of the Sulcis region! The view is great and for fans of Dante...it is the setting for Count Ugolino's cannibalism of his son. Climb up it, but try not to go in the summer...tooo hot!
There are some parts on the Island here, that are less exlpored and magical. It's hard to get there and pretty rocky, but if you get a mountain bike, you can head out the a pine forest that lines the cliffs of the far end of the island. From here you can descend into the fragrant forest and reach the cliffs. Work your way down and take a little dip on the rocks. I guarantee you'll be pretty much alone.