Prague is compact and hilly in the right places, and in December that suits anyone happy to walk in the cold. The old centre is small enough to cross on foot in well under an hour, and the views from the hills are at their sharpest on a clear winter day. Finish each day on Old Town Square, where the Christmas market is lit by the time the sun goes, soon after four. Three or four days covers it.
Be on Charles Bridge by eight. At noon you will shuffle across it in a crowd, but at first light on a frosty morning the parapets are white and the castle above Malá Strana is taking the first sun. Then climb, though not the steep way. The paths up from Malá Strana turn slippery in a freeze, so ride the tram to Pohořelec, walk past Strahov Monastery and follow the ridge beside the Hunger Wall to the Petřín lookout tower. It is broadly level the whole way. Wear boots with real grip anyway, and gloves that work a phone screen.
Save Letná for the afternoon. Cross the river from the Old Town and climb to the terrace by the giant Metronome, where you look straight along the Vltava at bridge after bridge. Be up there by mid-afternoon, because the good light goes fast.
Give another day to Vyšehrad, the old fortress on its rock south of the centre, a short walk from its own stop on metro line C. The ramparts are free to walk. The cemetery beside the Basilica of Saints Peter and Paul holds the graves of Dvořák and Smetana, and from the walls the river runs back towards the city. Return along the embankment. It takes half an hour or so, and it is flat.
Old Town Square's market starts around the first weekend of Advent and carries on into early January, with a tall tree lit in the middle and choirs and folk groups on the stage from late afternoon. It is the main event, and in the evenings it is packed. Many Praguers prefer the smaller market on náměstí Míru in Vinohrady, which normally closes on Christmas Eve.
In the last days before Christmas, stands with tanks of live carp appear on street corners across the city. Carp is the centre of the Czech Christmas Eve dinner, and the seller will kill and clean yours on the spot, though some families still carry theirs home alive to swim in the bath until the 24th.