Austria, Slovenia or Czech Republic in July?

Hi everyone. This is my first post here and I'm hoping for some trip planning advice. My husband and I will be vacationing on the Amalfi Coast the first week in July. We will return to the US from either Munich/Salzburg, Prague, or Ljubljana on 7/13 (we haven't booked our return flight yet, but these are the options). That leaves us with 6 free nights after the Amalfi Coast.
We've been to Italy several times, so we'd like to spend those 6 nights somewhere outside of Italy. We're considering Austria, Slovenia, or the Czech Republic for those 6 nights. We plan to fly from Naples to either the Munich/Salzburg area, the Triestse/Ljubljana area, or Prague on 7/7, so we'll have roughly a week to explore one or more of these areas. We're in our late 30's/early 40s. We enjoy good food, nice scenery (particularly mountain scenery), walking, and light hiking. We're relatively active and we enjoy sightseeing, but we'd also like a few days to just relax. We also tend to prefer smaller towns to big cities. I'm guessing that there will be lots of tourists in the Amalfi Coast area while we're there, so we'd love to spend the second week of our vacation somewhere less crowded. Which of these areas, Austria's Salzkammergut, Slovenia, or the Czech Republic, seems to be the best fit for us? Are there any "must sees" in the area that won't be terribly crowded in July? A trip that involves both the Salzkammergut area and Prague is also an option for us, but that may involve too much travel time between cities. Thanks for your help!