I get the impression that Dubrovnik is a bit more of a year-round destination these days. It certainly shouldn't be crazy cold, and to be honest it would be a relief not to have the hordes of summer cruise ship tourists. On the other hand, sitting around outside eating and drinking isn't such a great option at that time of year.

One thing that would still be stunning, assuming that you got clear weather, is the Bay of Kotor in northwestern Montenegro, only about 1.5h drive away from Dubrovnik (and doable as a daytrip). Truly stunning place, which is more about the spectacular fiord-like scenery and mountains than about beaches and lounging - plus the steep hike up the walls of Kotor old town would probably be much more pleasant in cooler weather