In reply to Blue Straggler:
Naples is a fantastic city, but I'd suggest that a night and one day might be frustratingly brief, given all that there is to see in the city and the vicinity. One possible benefit of going there in December, though, could be that it wouldn't be as punishingly hot as it was last time we were there - in July! If you do choose to go there I can recommend the Palazzo Turchini as a comfortable and reasonably-priced hotel.
Turin in the winter can be a lovely, atmospheric place. With a bit of planning you should be able to "do" the key highlights of the city in an evening and a day. As well as the 'standard' attractions like the Duomo and the castle in Piazza Castello, you can spend a happy hour or so strolling around spotting locations from The Italian Job such as the Chiesa Gran Madre di Dio (aka "God's Grandmother"), the Palazzo Vela and the Lingotto car factory. A donder through Parco Valentino along the banks of the Po can take in a visit to the charming if not also frankly slightly bizarre Borgo Medievale. If the weather is cooperative you get wonderful views of the Western Alps from vantage points such as the top of the Mole, or the Superga (reached by a rickety old rack railway from the suburb of Sassi on t'other side of the Po). Alternatively, of an evening, you can sometimes get captivating glimpses of the mountains silhouetted by the sunset by looking anywhere from south-west to north-west along one of the main thoroughfares out of the city centre such as Corso Francia. And if you're spending an evening there you'll also likely be mingling with the the locals making their "passaggiata" around the social hub of the city: Piazza Carlo Felice, Via Roma and Piazza San Carlo.
Bottom line: for a short visit I'd recommend Turin. Naples and the Costa Amalfitana are worth doing as a longer trip in their own right.