This is one of the city's oldest neighborhoods, once the home of the very wealthy until the 1877 outbreak of yellow fever caused many to flee to newly developing areas north of the city center.
The heart of San Telmo is Plaza Dorrego, the city's second-oldest plaza (after the Plaza de Mayo). There is a great milonga held there every Sunday evening. It attracts a lot of great dancers and is deejayed by hosts who are fantastic steppers in their own right as they put on an exhibition in the opening half-hour. You'll have to take my word on this as I was feeling too timid that evening to take photos. Earlier on Sundays, in the afternoon, there is a weekly, makeshift outdoor market held curbside: 'La Feria del San Telmo' (11am-5pm). It's part artisanal craft fair and part flea market. There are some great finds here but folks should note that some of it is priced to tourists. As with most vendors, bartering is allowed- if not expected. My only purchase was from a man who was selling churros out of a basket on his bike. He even threw in an extra and made my day!
A few of the buildings on its edges still date from the colonial period. There is a decayed grace here, and folks who have been to Habana Vieja in Cuba will experience a certain deja vu. This neighborhood is one of my favorites, and I like it most during sunset to twilight when the buildings glow gold and their ornamental tops become silhouetted against the sky.
Amidst some of these old buildings you may find a great number of restaurants and music venues. My best recommendation for a Saturday night is to go for a big filet at El Desnivel followed by a show at one of the hippest places on earth, Torquato Tasso. Caveat emptor: they don't have Uber in BA so you'll have to call a cab.
Tango-themed bars make up much of the entertainment in San Telmo, in addition to the restaurants and cafes that have been in operation for almost 150 years. Calle Defense, lined with antique stores, runs from north to south and is the area's main street. It takes its name from the route Argentina's colonial army used to defend itself from a British invasion in the early 1800s.