I started this trip with family, but I was able to stay far longer than they were.
We spent Christmas together in Sorrento, with visits to Pompeii and the Amalfi Coast. Everything was quite pretty, and the water was swimmable though not warm.
The coastline of Amalfi was very pretty, though the drive itself was quite narrow and winding.
Pompeii was very interesting, and this was a great time of year to visit - sure the weather was cold, but the crowds were much smaller.
It's amazing how much survived.
Rome was the last city I saw with the family, and it was certainly pretty! The Colosseum was large, but the real impressive ruins were the Pantheon and the Roman Forum.
The Vatican was a very large church with quite a bit of a hike to reach the top of. It really was quite impressive, though I've since seen a few other churches that would give it a run for its money, such as the Cathedral of St. John in Valletta.
Rome was also filled with a multitude of other churches - while I'll only post a photo of one more that was certainly not the extent of them.
Florence was the first city I visited alone, and it was a great experience. The art museums alone were worth the visit, and generally the city was small enough to visit easily while still having a ton of things to see.
Rome was a huge tourist city. Florence was a tourist town. Milan was just a city - not one really oriented towards tourists.
The Milan Cathedral was impressive, but I can't recommend The Last Supper. It was done on dry fresco, and has deteriorated dramatically - the reproductions you've seen are quite a bit better than the original nowadays.
There was still a great selection of art museums in the city, and the Pinacoteca Ambrosiana was a fun visit.
Venice is a classic tourist destination. It is also a massive tourist trap. Earlier I said that it was good to visit Pompeii in the off-season, but for Venice I would say that it's a necessity. The city is beautiful, but even with off-season prices and crowds it's still tough.
I was able to attend an opera at the Teatro La Fenice to finish the trip, and I'm glad I did - another benefit of visiting in the off-season!