I would advise if you have time and you are in Palermo on a day that has great weather I would suggest paying the 5 euros to climbing to the roof of the Cattedrale di Palermo as it has an amazing view of the city and is a pretty damn unique thing to be able to do. The tours run fairly frequently and the groups are around 10 – 15 people so I would advise buying your ticket for the next tour before exploring the interior, I made the rookie error of exploring the church first and having to wait around for the next tour to start.