If you have 2 weeks on the ground, you could add another stop and some day trips.
I'm going to Poland and Hungary in September, before the cold weather sets in. If you haven't been to Italy, I urge you to put it at the top of your list. I think everyone should visit Italy. October would be better than September, less heat and fewer tourists - lots of tourists but not as many as September. The classic trip is to fly into Venice, then see Florence and finish in Rome. If you have 2 weeks on the ground, you could add another stop and some day trips. If you have 3 weeks, you could add the Amalfi Coast. If you can fly home from Naples, you could fit the AC into a 2-week trip without rushing.