Just stroll that main street down to the sea, drink spritz and birra all the way down, stopping for mortadella sandwiches and arancinis and however much limon gelato you can stomach, should take a few hours. Add an hour if you get some hash from the backpack African kids. My wife does the fancy part where the Prada store and all that is but I just like to walk and drink and have a toke. There is a fantastic restaurant on the ocean at some very fancy resort that was very memorable, I'll ask my wife what it was
My only problem was how expensive it is to get into the old churches. That's like my favorite thing to do and its 35 dollars to get into each of them! Never seen that before in all my travels outside of world renown cathedrals, which, of course..
edit: I would fly into palermo for the game, then rent a car and drive across Sicily, even for the day if that's all you have, and fly out of Catania. Sicily is such a gem