Greece once had its own currency, the Drachma.
On 1 January 2002, this was replaced by the Monopoly-money look-alike Euro, which is now in use in many European countries (some exceptions are Great Britain, Sweden, Switzerland and the most recent member countries of that great bureaucratic entity the EU).
Those Drachmas you have collected can no longer be used.
But how convenient when traveling around Europe, to have one currency! Although it is unlikely the euro will be considered as desirable as the US dollar in the immediate future (recent weakness of the dollar may signal a change in sentiment), it is widely welcomed by Europeans and all those traveling and doing business within the Euro-zone.
In Crete, it is a good idea to collect change for larger notes, before traveling to small villages or going to buy low-cost items, but especially in smaller out-of-the-way places. Otherwise, you may face quite a wait while the shopkeeper you are paying tries to find someone who can make a change.
In the towns, clothing, camera and other shops selling higher price products mostly accept credit cards, as do medium to high priced restaurants and hotels. Visa and MasterCard are the most-accepted.
However, note that "cash is king" in Greece, and credit or debit cards are not accepted everywhere - it is a mistake to presume that mid-range hotels will happily take your card.
Some businesses will offer a lower price if you pay in cash - some just ask for more if you pay by credit card and sometimes there is no difference! This may seem irregular - but as always, just ask and you'll find out! If someone is in business to sell their products or service, they will usually speak English, German or French - if that doesn't help, sign language always works - just be patient and enjoy the holiday...
The most useful card for purchases AND cash advances is Visa, followed by MasterCard. The Diners and American Express cards have more limited acceptance (Amex being the better of the two).
Travelers' cheques in the world's major currencies can be changed at all banks and larger hotels.
Cash can be obtained from credit and debit cards at most banks (the majority work with Visa associated cards, far fewer with Mastercard). There is an ATM at the port, on arrival in Heraklion (look for the light blue kiosk as you disembark), Chania (Souda), and also at all the airports. Virtually all the machines provide instructions in English and often several other languages. ATMs all issue euros.
If you need cash amounts over and above the amount an individual machine or your daily limit allows, you can obtain a larger amount over the counter showing your passport and credit card. [Tip: This process is not recommended at the National Bank of Greece, which happens to be the only bank for over the counter Mastercard advances - be prepared for a long wait!]
Carry cash - accepted everywhere! Carry travelers' cheques (accepted sometimes, some places) and a Visa card!
Rates for the exchange of currency vary little between banks (they are normally posted daily in the window) and some hotels will give you the bank exchange rate. For today's exchange rates, see currency converter.
Advice: Before you leave home - write the numbers of travelers' cheques on the form you will have been given by the bank you bought them from, together with the list of phone numbers to call in case of loss. Put this in a plastic bag, seal it and keep this well away from your passport, credit cards, etc - in the unlikely event you find yourself without the valuable travelers' cheques one day, you'll be able to get replacements locally and possibly even delivered to you while you're here.
Do the same with numbers of your credit cards, shopping cards, petrol cards, debit cards, passport, and driving license - include the phone numbers to call in case of loss. Again, unlikely you'll ever need these - but it's the cheapest insurance you'll ever find.