Cartagena is hot in January, and the sun has real bite, but this is the dry season. Rain is rare and a steady breeze off the Caribbean takes the edge off the afternoons, which makes it an easy month for a first look at the old walled city.
The first half of the month is Colombian holiday season, when hotels fill and prices climb, so book early if you're coming then and expect to share the plazas. In the last days of January the Hay Festival usually brings a long weekend of writers and talks to the old town; book sessions as soon as the programme is out.
Do your sightseeing early and late. The Castillo San Felipe de Barajas is a short walk across the bridge from Getsemaní and far pleasanter at opening time than at noon. Bring light cotton or linen, a hat and shoes with grip for the uneven streets. You won't need a jumper.
Is it safe? The walled city, Getsemaní and Bocagrande are where visitors spend their evenings, and the old town has tourist police. Keep your phone out of sight on the street and leave good jewellery at home. The FCDO's Colombia page is worth ten minutes before you fly.
For an island day, most boats to the Rosario Islands leave in the morning from the Muelle de la Bodeguita, just outside the walls, and come back mid-afternoon. January's winds can make the crossing bumpy, so if you get seasick, sit towards the back. Expect a dock tax and a national park fee on top of the tour.
Keep the last hour of daylight for the city walls. Most of the ramparts are free to walk, and the Baluarte de Santo Domingo, on the sea side, is a good perch for the sunset. Once it's dark, cross into Getsemaní and follow the noise to Plaza de la Trinidad, where locals and visitors fill the square and the stalls fry arepas de huevo, corn arepas with an egg cooked inside.