Calle Florida near downtown Buenos Aires is a great place to walk around, do some shopping and have lunch. It is a pedestrian street that is closed off to traffic and is lined with restaurants, stores and street performers. The pedestrian section runs from approximately Avenida de Mayo to Plaza San Martin. It is easily walkable from one end to the other and a fun spot to people watch.
Calle Florida has many stores lining both sides of the street and is an excellent place to go shopping and enjoy the favorable exchange rate. There is a store called Vetement that carries a number of luxury brands. This is the store where I got a suit, shirt and tie for around $300USD. Very very very good deals. Galerias Pacifico is one of the nicer malls in Buenos Aires and also has a Polo on the ground floor.
One of the best things to do on Calle Florida is enjoy all the street performers. There are tons of them and they are more unique than the street performers you would see a lot of places in the states. There were two people that had wires in all the clothing to make it look like they were caught in a windstorm that would freeze in place for a few minutes at a time. Apparently these two are a staple act of Calle Florida because there were pictures of them being sold in various locations around town. There was another guy dressed like a robot who's costume was really thorough. In addition to the unique there are many musicians and other performers.
There are a number of "cambios" or currency exchange places all over the street here. It is best to stay away from these as a lot of them are slightly unreliable and likely to take a higher percentage of your money then they should be. If you do choose to exchange money at a cambio you will need to have your passport with you so keep that in mind.
Florida is also a great place to get a snack or some lunch without spending a long time at a full sit down dining establishment. There are lots of good cafes lining the streets offering cheap but good eats. Calle Florida is a must for a fun afternoon if you have some time in the city.