Stephansdom, the Graben and Hofburg courtyards, a coffee-house break, then the Rathausplatz market after dark.
Day 2
Imperial Vienna
Schönbrunn Palace in the morning with its courtyard market, then the Kunsthistorisches Museum or Belvedere.
Day 3
Neighbourhoods
Naschmarkt, the Secession and Neubau’s shops, ending at the Spittelberg market in the lanes.
Day 4
Day trip
Melk Abbey and the Wachau, or Bratislava’s old town and its own Christmas markets.
Day 5+
Go deeper
Leopoldstadt, the Riesenrad and Hundertwasserhaus, with a concert or standing-room opera in the evening.
Honest thought: daylight is short, so start sights at opening time and save markets for dusk. Two days give you the highlights; five let you add the Wachau and a night of music.