Banco Sur is open 24 hours and changes cash and traveler's checks.
ATMs are plentiful in towns and cities.
VISA is the more common, Mastercard less so, and Diners and Amex hardly at all. American Express are the most widely accepted travel checks and can easily be changed at banks. Other brands of travel checks are treated with suspicion and holders may have some difficulty getting them changed. The U.S. dollar is the second currency of Peru and most businesses will accept them. Be aware of the correct exchange rate if you expect to receive change.