Reserva Ecologica Costanera Sur
Astride Puerto Madero, this is the only ecological reserve in Buenos Aires, and the most accessible place I found in the city for getting some fresh air and walking through its 360 hectares.
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More information:
http://www.buenosaires.gov.ar/areas/med_ambiente/reserva/
http://www.buenostours.com/costanera-sur-ecological-reserve