TGI's at Acropolis, free refills and great brunch food, well if you like fingery messy, fast food. It's freezing though so take a jacket, or sleeping bag.
This is horrid advice. Who the hell has brunch at a glorified fastfood place??
OP, do yourself a favour, go to any of these delicious places:
Any Cuchara de Madera
La Francesa on Lincoln and Paseo de los Locutores
La Esquina de Tejas on 27 de Febrero on the Evaristo Morales side
Cafe El Conde in the Colonial Zone
Hotel Ovando in the Colonial Zone
There's a place on El Conde that has amazing crepes but I forget the name
Casa de Madera was a good brunch place, but I think it's undergoing renovation
There's a bakery at NovoCentro that has surprisingly good food as well for Brunch
There are so many other options, it would be a shame to do the boring thing and go to Friday's, I mean really.