For one weekend a year, in mid-December, the centre of Leiden hosts a floating Christmas market on the Nieuwe Rijn canal. Around fifty stalls are built on flat-bottomed barges moored along the quay, selling traditional Dutch Christmas food, mulled wine, and crafts. After dark the canal-side trees are lit and the reflection in the water is the photograph everyone takes.
Check the date in advance — it varies but is always close to the third weekend of December. Expect crowds; come at dusk for the lights, late evening for the easier pace.