Salzburg, Austria. My wife and I spent 10 days there biking this August…undoubtedly one of the most beautiful spots (the city but even more so the surrounding valleys) I’ve ever seen. And I’ve been fortunate to have travelled to many places.
If your girlfriend likes Sound of Music, this is where it was based (but I wouldn’t waste money on the tours). It’s also All Mozart, all the time. Nice city to walk in, the fortress is really interesting with great views, and decent food.
Rent a car, and get out to the valleys. If you have the $$, head to Werfen (about a 30 minute drive) and have dinner at Obauers, one of the best if not the best restaurant in the entire country (2-star Michelin). The castle in Werfen is incredibly situated among the sharp peaks of the Alps, and has a neat falconry show.
There are many other small towns of interest within an hour’s drive as well.
I loved Austria.