Best place to learn Spanish in Latin America?

I'm a teacher looking to hone my conversational Spanish over the summer and I'd like some input from experienced travelers on cities (and even language schools) to look at. My fianc�e would be travelling with me and we'd like to avoid Mexico as she lived there for much of her life and we've traveled in that county a good bit over the last few years. Also Spain (or at least Madrid as she lived there for part of her life also) is somewhere we'd like to avoid. A location with some history, either colonial or pre-colombian, would be great. My fianc�e would love to be near the beach, and obviously we'd like somewhere safe. We've kicked around the idea of the Dominican Republic; Cartagena, Colombia; or maybe even Equador. Thoughts? Thanks in advance for any suggestions you can offer.