Santa Marta is the base for three quite different trips, and almost every agency on the Carrera 3 sells all of them: the four-day walk to the Ciudad Perdida, the beaches and jungle of Tayrona, and the coffee fincas and birding around Minca in the hills behind.
The Ciudad Perdida operators are the ones to look at carefully — the trek runs on a fixed price, and the difference between companies is the guiding rather than the cost.