Since strikes and power outages are not uncommon in Greece, always keep extra cash on hand. Most banks and currency exchange offices are clustered around Syntagma and Omonia squares. Many change money only until 2 PM, even if they stay open later. Commissions tend to be between EUR 2 and EUR 3 but can be more at an exchange office rather than a bank. American Express cashes American Express traveler's checks free of commission.
American Express. Eurochange. National Bank of Greece.