Santa Marta's night is small, cheap and packed into about six blocks of the old centre — one pedestrian square, one bar street, and hostel rooftops doing a lot of the heavy lifting.
Parque de los Novios
The pedestrianised square at the centre of the historic quarter, ringed by restaurants and bars whose tables spread into it. It starts as dinner around 8pm and drifts into drinking; nothing needs booking, and everything else worth doing at night is within a few minutes' walk.
La Brisa Loca
A party hostel in a restored colonial building at Calle 14 #3-58, with a rooftop disco open to non-guests plus a pool and bar downstairs. It is the reliable fallback when the rest of the centre is quiet — loud, young and mostly foreign, which you may or may not want.
Carrera 3
The pedestrian stretch of Carrera 3 running off Parque de los Novios is the centre's bar street: small open-fronted places, reggaetón and vallenato, drinks in the low tens of thousands of pesos. Busy Thursday to Saturday and noticeably empty the rest of the week.
Taganga
A fishing bay fifteen minutes north over the hill by minibus or taxi, with beach bars that run late and a long-established backpacker scene. It has a rougher reputation than the centre, so travel both ways by taxi rather than walking, and keep track of your things.
What the night is actually like
Santa Marta is hot, humid and low-rise, and the night follows: almost nothing indoors or air-conditioned, most bars open to the street, and things winding down around 2 or 3am rather than 5. If you want a full club night, the coast's answer is Cartagena or Barranquilla, not here.