What is the best way to get to the Barranquilla airport if I want to stay the night in Santa Marta? Would this be unreasonable or too complicated?
You can go by bus from Sta. Marta to Barranquilla very easy, it´s a frequent 1,5 to 2 hrs. drive I guess and you have the choice between different operators.
There´s a bus terminal in Sta. Marta, you should go there by taxi (for only 2 or 3 dollars within Sta. Marta). The busses drive to a terminal in the south of Barranquilla. From there you could take a taxi, or you ask in the terminal for a bus-connection to the airport.
Taking a taxi for about 100 km´s from Sta. Marta to Barranquilla is maybe not the best idea. Even when the taxis are cheap in Colombia...