In Catania, in the fish market (la Pescheria), Trattoria La Paglia (Via Pardo 23) makes very good food. It's the first place I'd try. I also liked La Siciliana, in Viale M. Polo 52/A.
Lots of good places in Palermo, but I urge you to try
pane ca meusa (a sandwich made with spleen and a little caciocavallo cheese) at the eponymous Pane ca Meusa (Via Cala 62) or in one of the shops near the train station. For excellent Palermitani food, I like Capricci di Sicilia (Via Istituto Pignatelli 6). A great place to stop for some quick
sfinciuni or
panelle is Antica Focacceria San Francesco (Via Paternostro 58).
And perhaps pick up some of Andrea Camilleri's Inspector Montalbano mysteries to read on the flight over. The inspector is a serious eater.
Buon viaggio in Italia.