I found Salzburg in Austria to be such a charming city. You can get the Salzburg Card for either a day or 48 hours and it is the best deal I've experienced in just about all my travels: For $39 (the price slightly varies based on time of year), it gets you access to a lot of the major attractions around the city. I did the math and if I had purchased everything individually it would have been well over $150.
My personal favorites were climbing the Untersberg (where scenes from The Sound of Music were filmed), which gives you an incredible picturesque view of parts of Switzerland, Austria, and Germany, the Salzburg Fortress, and going to the Trick Fountains at the Hellbrunn Palace, which is an elaborate underground hydraulic system set up around 400 years ago and still uses a lot of the same mechanisms today to shoot water at unsuspecting guests. I'd also suggest taking a tour down the river—the boat you're on does a waltz as it heads back into the docks.
Wherever you go in Austria, get as many pastries as you can. They are plentiful and delicious. And it's very cliche, but the fondue I had in Switzerland was phenomenal. Have a lovely time!