Zell am See is a must see (along with Gross Glockner, a glacier where the winds are always ice cold). A rather unusual but must-see sight is Matthausen, a concentration camp near Linz -- quite a juxtaposition to an idyllic location near the Danube. Not quite what one expects to find in Austria (but to the credit of the Austrian government, it has been immaculately maintained as a memorial and warning). In the same vein there is a fabulous bike path next to the Danube (goes for a hundred miles), and along it is a medieval Catholic church filled with human bones (even the altar is decorated with human bone) ... I think the custom was to exhume bodies from the graveyard after a few decades, and then put them into the church. Finally the area of Austria SE of Vienna doesn't get a lot of tourists but has a certain charm (as does Graz).

As for luxury hotels, Austria has some of the best in the world (mostly in Vienna). The Imperial makes my "10 best hotels in the world" list. Salzburg is pretty nice, too.