In the morning, I went for a guided tour of the Festspielhaus.
In the afternoon, I visited the Domgrabungs Museum, Mozart's Birthplace, Residenz State Rooms, Residenz Gallery, Mozart's Residence, and Mirabellgarten.
By the way, I bought the 72-hr Salzburg Card (34 Euro) which provided for free public transport and free single admission to most city's attractions.