For us, visiting the National Aviary of Colombia was a highlight. It's kind of like a zoo but only features native birds. It's brand new and is about 15 miles from Cartagena, We join a day tour there, which included a rustic beach front lunch at Playa Blanca.
We were in Cartagena for 4 nights. Just soaking us the ambiance was satisfying. Cartagena has nice museums and great restaurants.