South America Calling!

I've been trying to think of where to spend my next summer...naturally! I have a month and a half off and I'm not about to waste it away in Virginia. I had planned to visit in Europe with my sister but with the dollar in the pits, and trying to live on a budget, that's just not going to happen right now. I do however, have some co-workers who are from Colombia who are going there for the summer and invited me. I don't have a ticket but it's looking like that's a definite possibility. And, I made a friend who also has next summer off and is looking for some adventures so we are tenatively planning a trip to South America at the moment. Colombia is on the list for sure and hopefully Argentina. I would love to hit up Peru too and Brasil would be ideal. My question to all you readers, who's been to South America and can tell me where I have to go? I'm taking recommendations!