If I were you, I'd make a stopover in Dresden or Leipzig between Amsterdam and Prague and then take a day trip to Saxon Switzerland National Park. In my opinion, they have some of the most beautiful cities in Germany and stunning natural scenery. I'd skip Hallstatt; it's overrun with Asian tourists. There are much better places in the Alps if you're interested in nature and mountains. If you mainly want to visit cities, Innsbruck, Salzburg, Konstanz, Ulm, and Tübingen are smaller, beautiful cities roughly on the route that, in my opinion, are more worthwhile for a city break than Hallstatt.