Occupying a good chunk of the block sketched out by Estados Unidos, Defensa, Carlos Calvo and Bolivar, the Mercado de San Telmo is a place which locals and tourists visit in almost equal numbers. The latter to buy antiques and souvenirs, the former for their day-to-day groceries.
The geographic center of Buenos Aires is Caballito, a charming neighborhood with large green spaces, and well served by the Subte. Although it's not on the top of the normal tourist itinerary, this barrio has enough highlights to make it worth a trip.
In 1994, Plaza Serrano was renamed to honor the famous Argentine author Julio Cortázar, but locals will look at you in confusion if you ask for directions to Plaza Cortázar. Everyone still knows the lively, circular heart of Palermo Soho as Serrano.