I've never been to the Cinque Terre that late in the year. Before you decide, check the historical weather patterns for that area at that time. If it's likely to be rainier, then maybe consider rural Tuscany.
It is a good idea to take a vacation from your vacation, though you don't have a huge amount of time. If you don't go to the Cinque Terre, possibly a smaller town in Tuscany, such as Montepulciano, Pienza or San Gimignano would give you a change of pace.
Also, considering that you only have 11.5 days (how many nights? And does the .5 include your arrival or departure day?), factor in travel time. It will take longer to get to the Cinque Terre than it would to go to Siena or other places in Tuscany. Are you willing to give up time on your vacation for that extra travel time?