If you want a real local Costa Rican experience, this is it.

If you want to eat McDonalds and speak English, do not come to Puntarenas. During all my travels around Costa Rica (which Puntarenas's central location makes very convenient) I did not go to one city that was as authentically Costa Rican as Puntarenas. It is one of those little gems that has seemingly escaped the rampant Americanization that has hit the rest of Costa Rica. You will find few English speakers and no McDonalds in the city, but what you will find is a rich local culture and miles and miles of beautiful beach. If you want to guarantee that you'll learn Spanish, I couldn't recommend Puntarenas any more highly. There is no way no not interact with locals and I would say 90% of the program made local friends with whom we all continue to keep in touch with.

If you want to learn Spanish and feel like a local, then choose Puntarenas.