You've toured Italy before, so if you want something completely different, this might not help, as I've only been in Europe on Dec 25 in Sicily, but . . . although it can be rainy in Spain and Italy in December, it's not sweltering like it can be in summertime. It might be worth considering Southern Europe at Christmastime, if only because seeing it at other times of the year can be so hot. We were in Rome the week before Christmas, and while there were visitors at sights, we never were elbow-to-elbow, and didn't have to stand in lines. We'd arranged a Rome apartment for a week well before we left the USA, so can't say for sure what your experience would be getting a room. The city had a great holiday air, with a lit Christmas tree outside the Colosseum, presepi around nearly every corner, and just about everyone toting a panettone home in a yellow box by the ribbon handle on top. Central Rome was very pedestrian friendly, except for sidewalks where people had walked their dogs but hadn't cleaned up after them :-P , and sometimes we seemed to be the only non-smokers out walking.
If you went to Spain, how about Madrid or Barcelona? Bilbao (Guggenheim Art Museum and tapas in the Casco Viejo) in the north? Seville or Granada in the south?
Are you also flying last-minute, as that could really affect your airfares?
