Do you plan to rent a car? Cefalu is very doable by train, and you can reach Castellammare del Golfo in about 2 hours, but I'm not sure that will work so well for the other places you mention on a short trip. Trapani is over 4 hours from Palermo by rail, and the public-transportation path to Erice is via Trapani. They're close, but if you don't toss a taxi into the mix, it might take an hour to get to Erice.
One thing to keep in mind is that in Sicily, Sunday/holiday bus service is thin to nonexistent, and trains don't go everywhere, at least not by a direct route.
Palermo is definitely a place worth multiple days, and if you're flying in from North America, the arrival day doesn't really count.
I was in Sicily in June (not my brightest decision ever) and don't know what the small coastal towns and Erice will be like in December.