Is the Cinque Terre "open" in the third week in December? Should we by-pass the towns or try to see them?
I see you're from Seattle, so you know the particular joys of a winter weekend at the coast when it's wet, windy and most things are closed. Under such circumstances, I think the CT would be beautiful but slippery. A lot of the residents use the money they make during the tourist season to do their own traveling to places like the Seychelles.
In short, it depends on your other options. You might take advantage of the time of year and research where there might be cool Christmas celebrations.