Taxi rates are usually posted at the hotel. Or, you can get the doorman to negotiate for you. The fare for a return trip should be the same as getting to your destination. You can negotiate too if you speak a little spanish.

I would keep some US$$ for tips..and depending on your package, you may need the departure fee of about $20 in US funds. If you are tipping people like housekeepers in the hotel, they appreciate US$. The casas de cambio usually have better rates than hotels, and lower service fees....you can also use ATMs-they are everywhere. have fun