I recommend you contact Gaston Cernadas via his website http://www.bafreetour.com/en/index.html

He and his sister do two different great free walking tours of the historical sites and architectural sites in BsAs, which are a lot of fun. They are also available to schedule private tours. Really nice young people who speak great English, with a good sense of what visitors like to see. I'm not sure what they would charge for a private tour, but I imagine it would be affordable.

If you hook up with them, tell Gaston and Sol "hello".

I highly recommend going to the San Telmo artisans market on Sunday afternoon. You can get there from Plaza de Mayo (near Casa Rosada palace) by walking down Calle Defensa. Walk on down to Plaza Dorrego, stopping for food, drinks, and ice cream along the way, as well as all the wonderful street performers and tango orchestras. A great way to spend an afternoon.