Cartagena, on the Caribbean cost of Colombia, about 2 1/2h by plane from Miami and less than 1h from Panama is one of the most beautiful colonial cities of this planet. One of the main ports during the time of the Spanish Empire Cartagena has a divine old town with an old city wall, an enormous castle, picturesque streets, stunning cathedrals and buzzling squares. The city is made up of different islands with water all around and in just 45min you get to the nearby coral reefs and Rosary islands. The colorful mixture of people, fruits dances and rhythms is enchanting and is very likely to bewitch you as well. Having the tranquil and gorgeous city of Cartagena as your base where you can learn Spanish, Salsa, dive, windsurf, kite-surf or just hang out at the beaches there are plenty of attractions just day-trips away: The National Park of Tayrona close to Santa Marta, the desert of La Guajira, Rafting and Abseiling in San Gil close to Bucaramanga, Medellin, the Coffee Zone or a bit further Cali, one of the capitals of Salsa worldwide.
Cartagena offers several backpacker hostels with a lot of charm and well-maintained and there are all kind of hotels - until the most luxury ones you might be able to imagine. The spectrum of restaurants is amazing and there are quite a lot of night clubs.
Cartagena is an absolutely safe city with all services you might want to look for. Just be prepared that Cartagena is not one of the cheapest cities you might come along on your travels.