One of the long-standing Santa Marta operators, open Monday to Saturday 08:00–18:00 and Sundays and holidays until 17:30, which is worth knowing when you arrive on a Sunday afternoon wanting to leave on Monday morning.
Ciudad Perdida in three, four or five days, the Tayrona beaches, Minca in the coffee hills, and the Guajira. The four-day trek is the standard COP 1,860,000. Note that the English-language version of their site prices the same walk in dollars at a figure that does not convert back to the peso price — if you can pay in pesos, ask for the peso price.
They split their WhatsApp into four lines and label each one: bookings, corporate, conferences, and general. Use the general line unless you are booking a group.