To add to Mac's suggestions, with the week you have I'd recommend either a combination of Fes & Chefchaouen OR Marrakech & Essaouira. The first gives you an interesting medina experience combined with a very pretty mountain town and the Roman ruins of Volubilis between. The second a large city with all it has to offer plus a much smaller and very beautiful seaside fishing town with good tourist infrastructure. For that time of year I'd recommend Marrakech/Essaouira.
If you're passing through Europe on your way you can likely bypass Casablanca entirely and fly one of the low cost carriers directly into Marrakech.