I second Rojas. If the classes don´t match up with your schedule (they start 3x per year) I would take any class that encourages social interaction, especially theater in Spanish. There are some classes made for non-native speakers, but if you can, find one that is for native speakers. It was the best thing for my Spanish, and people like to grab a beer afterwards.
There are tons of classes on tons of topics, find one at night that lends itself to discussions, or a book club. You can also join a sports team or some sort of volunteer activity, or running club.
If you want more social options, go to a bar that has some expats but not too many, there will be Argentines looking to do "cultural exchanges" (hooking up with foreigners). if you go with a friend from the US, go with one, not in a group. That will encourage people to be more open.
And choose to be alone. Go sit a cafe alone, go to the movies alone, museums alone, so you don´t get caught into a group dynamic, and you improve your Spanish.
