There are some who go to Ibiza for the parties, and there are some who go for the World Heritage Site; count me in the latter group. When I was looking for an easy day trip from Barcelona in spring 2013, I settled on Ibiza since it was only a short flight away. Over 2600 years ago Ibiza was settled by the Phoenicians, who named their new port after the Egyptian god Bes (fittingly the god of music and dance). The Phoenician settlements and the fortified Dalt Vila, or Upper Town, in Ibiza Town form the cultural backbone of this mixed heritage inscription. On my visit, I spent a good couple of hours walking around the Upper Town ramparts and learning about the city's Arab, Catalan, and Spanish heritage. Unfortunately the cathedral was closed, but the views of the town, harbor, and rocky coastline from the fortress walls were impressive. Ibiza's natural component of its mixed heritage inscription is for coastal marine life supported by underwater seagrass. While I didn't get to see this component up close, I was very impressed with Ibiza's natural beauty, particularly on the rocky north shore near the Port of Saint Miquel, which, although not part of the inscribed area, was extremely photogenic.
Logistics: Ibiza is accessible by boat or plane, and I found it relatively easy to travel around the island via the local bus system.