Every Sunday from about 10am to 5pm, the plaza at the heart of San Telmo fills with antique stalls, leather goods, old books, and the kind of Argentine curios that are hard to find elsewhere: vintage football shirts, mate equipment, pre-Perón political posters, silverware from estancias long dissolved. Quality varies considerably, and prices are negotiable.
Tango couples perform in the surrounding area for tips, and the surrounding bars (the ones on the square itself tend toward tourist pricing, but those a block away are more reasonable) fill up as the afternoon progresses. This is the best place in the city to watch street tango being danced by people who actually know what they are doing rather than performing for cameras.