It is definitely worth it to check San Telmo open market. I went on a Saturday, and it was very interesting. You find all around a huge square an open market, full of stands. Basically they sell used articles, from books to hats. A flea market. You can see a lot of interesting things there to buy. Plus there are all the people dancing Tango at the street, and the outside cafes, very relaxing.
La Recoleta, is one of my favorite places to hang out. The city grew around a cemetery. It is huge! I loved the little telephone cabins they have there. It is also where you will find Hard Rock Cafe. Very particular, you can see all around Buenos Aires, ladies walking dozens of Pekingese dogs at a time.