At Centro cultural Kirchner ( CCK), the largest cultural center in Latin America, there are cultural, music, dance and art events held here and in most cases entry is free!
CCK is very popular among locals and tourists and events get fully booked way before the D-day. Book in advance though it's not necessary for all events, during some events the attendance is not that good and you get a seat even without advance booking.
You can get your ticket here
http://cck.gob.ar/calendario
There are around 36 other cultural centres within town that run a similar program. You can go to those in case you miss a spot at CCK.