Buenos Aires is a city of cafés. The tradition goes back to the 19th-century Italian immigration wave that brought the espresso culture with it; the result is a city where sitting in a café for two hours over a single cortado is entirely normal and not requiring of additional purchases. The oldest cafés have Relevant architectural detail — pressed tin ceilings, marble countertops, painted walls — that makes them worth visiting as spaces as much as for the drinks.
The bar scene is mostly nocturnal. Bars open around 8pm and get going after 11pm; clubs don't start until 1am and continue until 6 or 7am. Palermo and San Telmo have the best bar concentrations; the Microcentro empties after 8pm on weekdays but has some good traditional bares about which the locals know.