La Casa Minima

When you visit San Telmo, make sure you pass by the Pasaje San Lorenzo, on number 380, you´ll see whats considered the smallest lot in Buenos Aires. called "Casa Minima" or "Casa del Liberto", it is said to have belonged to an emancipated slave, that made his abode in a small lot that was vacant.

Basilica del Santísimo Sacramento

The church of the Blessed Sacrament in Buenos Aires is a most visited landmark of the city. Kinda hidden behind an Empire State lookalike building, its actually very well located in the Retiro neighbourhood , Being one of the most lavishly decorated churches in the city with gold and silver Works, a French organ, important stained glass Works, marble and blue granite, bronze and Venetian mosaic ornaments. 
It was built by the Anchorena family that lived basically across Plaza San Martin in a mansion being used today as the Ministry of foreign affairs headquarters, that can also has a tour guide available.

The idea was that the church´s steeples could be seen from the Anchorena´s house, until a marriage proposal was rejected by the family´s matriarch because of lack of social status. The aspiring bride belonged to the Cavanagh family, very rich, but nouveau rich. So it is rumoured that in revenge, the concrete behemoth called the "Edificio Cavanagh" was maliciously placed blocking the church´s view.

"El Ateneo" Bookstore

"El Ateneo" is a bookstore with several locations in Buenos Aires. The most famous one is located at Av. Santa Fe, half a block away from Av. Callao.
This former Theatre-Cinema was called the "Grand Splendid" and still has the sign on its facade. Converted to a bookstore in 2000, it has kept its original decor as it was adapted into a bookstore.
Besides admiring its interior you might want to pick up a book on Buenos Aires as a souvenir.

The Guardian ranks it second nicest bookshop (as the brits say) in the world.
http://www.theguardian.com/books/2008/jan/11/bestukbookshops
 I´m attaching the article cuz you might want to check out the others if you´re in the mentioned cities.