Heidelberg would be a good choice. It is more or less in the middle (between Florence and Amsterdam). It is a lively town with a great castle. I visited Heidelberg last September and loved it. A good place to spend a day or two, in my opinion. There are a few small towns with great castles on the Rhine such as Burg Eltz, or Marksburg. These could be an option as well. Else, you could visit one of the towns in Bayern such as Nürnberg or Würzburg (great architecture), though these are slightly out of the way.

You could swing a little bit left and hit up Paris, Brussels, etc. on the way back to Amsterdam. Also, the french riviera, like Nice, etc.

Take a night train north, out of Florence, and wake up in (I know that you don’t want to go to France, but) Paris.

My first time in Paris was wonderful. I planned three days in Paris because one of our friends refused to go with us to Europe if we didn’t schedule time there. I made the time, he didn’t make the trip. I did not want to go there. I had no interested in France, French, or Paris. But, oo-la-la, WOW.

I have been back twice. Amazing.

Other option: Take a night train out of Florence and head for Baden-Baden, there go to Friedrichsbad (http://www.carasana….). It is a unique and relaxing experience. I told my wife afterwards that I had not been touched like that since my mama washed me in the sink! You will never be cleaner than you are when you walk out their doors after 4-5 hours of personal spa treatment. Even Mark Twain enjoyed the baths, “I fully believe that I left my rheumatism in Baden-Baden. “ Located between Strasbourg and Stuttgart in a national forest.

Otherwise go to Switzerland. Pick a mountain, find a town, love it. Love everything about it.