Should I say Monreale is 'off the beaten path'? I hope not, as Monreale's Cathedral is the most important single masterpiece anybody should visit while in Palermo. Monreale is just 15 km away from Palermo, on the hill above the city, it can be reached very easily by urban bus. I'm very sorry I have no printed pictures of the interiors, which are among the most perfect examples of byzanthine mosaic still exisiting, here's a worth browsing site: www.casaspasia.com/monreale3.htm.
You may be able to notice Monreale's and Palermo's Cathedral are one the opposite of the other: while in Palermo the exteriors are incredibly decorated and the interiors are poor, in Monreale you will notice the opposite priority. The fact is explained by the distinct task of the two cities: Palermo was the seat of the Emperor, the civilian power, while Monreale used to be the seat of the Archbishop, the spiritual power.
The picture I used above is extracted from www.cittadimonreale.it, which is intended to be the official site of the municipality, but - unfortunately - it has big HTML problems, so I decided to use their picture directly.