Casablanca is huge and not really a touristy city. Agadir is a nice beach city, lots of restaurants along the beach, some nice souks and a good fish restaurant. Don't know how cool it will be there in December though. Two smaller cities, Assilah and Essaouria are walkable and picturesque, Essaouira is larger and has an ancient bazaar and thuya wood items, argan oil, and a lot of art for sale. Assilah is smaller and has a nice outdoor restaurant area, small shops, a beach, etc. Marrakech is more lively, especially Djma El Fna, although probably colder than the coastal areas. Tangier would probably be the best place to stay if you are going to Assilah for a day trip, and Fez and Volubilis is near by.