Old Town Square and the Astronomical Clock early, the Jewish Quarter, then the Municipal House and the market lights after dark.
Day 2
Castle & Malá Strana
Prague Castle at opening time, down through Malá Strana to St Nicholas and Kampa, then Charles Bridge at dusk.
Day 3
Cubism & the river
The House of the Black Madonna, the Cubist lamppost and Vyšehrad’s Cubist villas, ending at the Dancing House.
Day 4
Day trip
Kutná Hora by train, or Dresden for its famous Christmas market.
Day 5+
Go deeper
Villa Müller (book ahead), Letná and the Trade Fair Palace, or Vinohrady’s cafés and Plečnik’s church.
Honest thought: daylight is short in December. Start early, put indoor sights in the middle of the day and save the markets and bridges for the evening, when the lights are on.