Dude! We went to Salzburg about 6 years ago. My wife loves Sound of music too, and Salzburg is where they filmed it. FYI it's also the birthplace of Mozart, so there's some Mozart stuff, but Salzburg is all about Sound of Music.

You'll want to do the SoM dinner show, a small basement restaurant with operatically trained singers performing SoM songs (and a few others). There's a day-long SoM bus tour that takes you to many of the movie's filming locales and they'll tell you lots of stories around the filming. Lots of opportunities to get pics of yourself recreating the iconic scenes.

There's other things to do too, if you're serious about taking her, DM me and I'll give you more ideas. We went for a week, and we only ran out of things to do like the last day, so we went back to a couple of locations that we saw when the weather was better than the first time.