I was in Cartagena on january, more focused in the islands, beaches. But at night I did the typical walks around the historic center. The cafes in Santo Domingo square are nice, with musicians playing around.
Not sure about your interests, but if nightlife is included I recommend Mister Babilla. It's a cool club you will enjoy a lot. It's convenient to reseve or arrive not so late cause it gets full.
www.misterbabilla.com