During our time here in Buenos Aires, we've really enjoyed all the parks that abound in the Palermo barrio.  One which is just a quick walk from our house is the Botanical Gardens near Plaza Italia on Avenida Santa Fe.
Opened in 1898, it's comprised of almost 20 acres, 5,500 different species of plants and packed with tons of very mature trees. Along with the greenery, the garden also has 33 well known sculptures and monuments. It's a perfect place to lounge on a secluded park bench and study your Spanish flash cards.