You can't leave BA without having gone to a Milonga. This is a great space because is tourist friendly but is full of some of the best tango dancers in the city. If you want, you can learn some tango steps before the milonga and then try them on the dance floor. If not, you can always get a table, watch the spectacles, and have some drinks (or even food). They have a tango orchestra, and they invite great dancers to perform. You'll get to see how BA people lives the night, usually whit a beer or a wine glass in one hand ;)
The place opens every day (except Sundays) from 21 to 05hs, but tourist usually leaves around 01/02.
Perú 571, C1068AAA CABA, Argentina