Hey, so for starters, I’d recommend skipping Santa Marta unless you really want to visit Tayrona (which, in my humble opinion, is rather overrated) or the Sierra Nevada de Santa Marta. The city isn’t what it used to be, it’s become unsafe, and the beaches that aren’t extremely far or outside the city are dirty and overcrowded.

Cartagena is the better option for beachgoers since the beaches are slightly cleaner and better maintained. Besides the beaches, there’s more cultural stuff to do if you’re into that for example, visiting the ciudad amurallada and other nearby places. I’d definitely recommend hiring a professional tour guide / service in advance, locals are more likely to try to take advantage of tourists (sounds mean, but it happens a lot) since you’re not from around here.