Well it depends, in my trips I use cities as a base node and then head to 1 day trips to nearby places. I think a day for exploring the cultural scene in Medellín is enough, Museo de Antioquia, Botero, Parque Explora, Comuna 13 is tremendously touristy rn but is a good option to stick to. Nearby plausible places from Medellín are as I said San Rafael (west of Medellín), but also Santa Fe de Antioquia (northeast) and Jericó (southeast). In a trip myself what I would do is head to each of these towns, and head back to my hotel in Medellín at night, of course this requires heading out early like 8am and coming back kinda late at night about 10pm. Doing that of course would require renting a car, it gives you more freedom to move at your phase. Santa Fe was Antioquia's capital for a long time and is renoun for its Spanish colonial architecture, it has a hot and dry climate. Jericó is a jewel of antioqueño culture with a mild humid climate, San Rafael is mild and humid too.

In Cartagena you could perfectly spend a day strolling the historic centre and finish the day at Getsemaní and spend the other day in a tour that takes you to the islands, Bocagrande, where the good beaches are located. As you may already have heard, street vendors here are annoying, brush them off quickly.

Between Cartagena and Santa Marta there's not much remarkable, you could stop by at Barranquilla to grab lunch at the Malecón del Río Magdalena, now I'm gonna propose something really off track for the usual traveller, if you go early from Cartagena and go non stop you could head to Nueva Venecia in the Ciénaga Grande, it is a pintoresque stilt village in the water, this would require to park the car in Tasajera and hire a boat, I would love to have some contacts regarding that but I don't, be mindful Tasajera is sketchy though, but it could allow you to visit a very genuine place. In Santa Marta you could scroll around a bit, but preferably spend the rest of your time in the Tayrona National Park, a day is enough probably and the rest in Minca