Lots to see and do
Massive city with so much to look at. San telmo is a really nice area, with a market on sundays. You'll find food, music and loads of gifts to take home. Its also nice just to sit around in the park if the weather is good. A Tango show is a must as you wont get a show as good anywhere else in Argentina.
Insider Tip:Combine steak-eating with a tango show at "boedo y San Juan". I'd love to go again!