Where to stop on the way back to Italy

I'd suggest traveling to Barcelona first, as it is easily flown to from the U.S. It's an absolutely great city, too.

You can get to Milan inexpensively from Barcelona on EasyJet or Vueling Airlines.

I would have to agree with David on this one. While any of your three choices would fit the bill as far as history and art, Barcelona would have the edge for ease of travel from US and from there to Italy. For ease of travel Vienna would be next and Prague would be last. If you're drawn to one more than the others then maybe the less convenient travel would be worth it. I've always ended up going to the places I was drawn to based on research and recommendations from others, not those most convenient. But that's a personal choice.

Three - nil. Barcelona. At its best in post summer glow of September.

Use rome2rio.com to see your options for each city. They are all worthy of 3 days.

If you are partial to train rides through fantastic alpine scenery, you could pick Vienna and arrive in Italy via Innsbruck. It would take awhile, so you would have to value the train ride vs time in one location.

Ok now that sadly my #1 choice of Prague has been eliminated, because I'm not spending my vacation flying all over Europe to get from there to Florence, my wife put London into the mix. So might I get your thoughts on London or Barcelona as either is easy to move on to Florence or Milan from. Thanks in advance,

In my experience London is the easiest destination to get to from US and has the most budget flights to other parts of Europe so it's definitely as easy or easier than Barcelona. It would really then depend on which has the sights that you want to see/experience. If you've never been to London then I would personally recommend that option as there is so much to see and do there, although 3 days is barely enough to get a taste of it.

I almost always stop in London, sometimes both coming and going. In September, the Globe Theatre is still doing performances - not sure if tix are still available, but you can check that out on their website.

Also, look at London Walks and their Day Trips out of London. I did a visit to Stonehenge and Salisbury with them one September in time for the solstice. They have lots of other interesting destinations and walks, too.

Check Kayak.com to find out where flights from London depart for Florence and Milan. I think you will find a greater selection flying to Milan, especially from Heathrow.

I would look for either the cheapest direct flight from Boston ( or an upgraded seat on one) to Europe and visit that city. I would also consider the ease of connection via a budget airline to my next destination. That will probably restrict you to several European hub cities that fly to Boston. Consider London, Amsterdam or Paris

Just returned from a 33 day trip; my vote goes to cities Verona, Italy and/or Salzburg, Austria, both are gems.