If you are bringing cash, try to use denominations lower than $100. There are a couple series of $100 Bills that WILL NOT be exchanged**.
This was due to rampant forgeries. If I can find the specific series or if this has changed as of late, I will repost.
If you end up having a series of bill that can't be exchanged at a bank or cambio, you can go to the airport and exchange it with one of the "street" cambio guys (I don't know what they're called). You will have to negotiate the exchange rate with them, so be aware of what the current rate is. They'll try to take a big haircut if they can get away with it.
**This happened to me in 2008, so the situation may have improved by now.