Puerto Peñasco is very friendly to the US dollar – most merchants post prices in USD, most restaurants accept cash and credit cards – so more often than not you’re OK heading to Peñasco without a fistful of pesos.
You’ll probably wind up with pesos as loose change, however, and if you pop into an ATM, it is most likely going to give you pesos. You’ll also see ‘casas de cambio’ here and there, little tiendas that do currency exchange – for a fee.
Also – if you’re using your credit card, be prepared that your issuing credit card company may add on a nice little ‘Currency conversion’ fee of about 3 to 4% on top. Check this Currency conversion costs chart out to see if your bank does, and consider just using cash when you are buying food, drinks, etc. It may save you in the long run.
Most folks who charge credit cards show the peso amount first, with a smaller peso to USD conversion rate and USD equivalent on the bottom of the receipt.
Here’s a historical chart of the peso-to-USD fluctuations from finance.yahoo.com: