Cash

 
As a rule: cash rules. Greeks like cash money and my personal guess is that they like it for tax reasons. It's difficult to trace. Since there is very little crime in Greece carrying cash is not particularly dangerous. I always carry a few hundred Euros when I travel around. You never know where the next working ATM is and if there is one at all at your destination.

 
ATM

The number got up big time. At this stage you'll find an ATM in every small town. They have a tendency to break down on Friday evening and they will only get fixed the Monday after. Never rely on them too much. In remote villages the next ATM can be 30 or more kilometres away. Always keep a sufficient amount of cash as backup. ATMs accept most credit cards and debit cards.
 

Traveller cheques

They are safe but you pay fees for buying them and selling them. The fee is pretty low in Greece. The last time I cashed a traveller cheque it was 2%. You can cash them in hotels, travel agencies and car rentals. I suggest sticking with well known brands like American Express, Visa and Thomas Cook. You might run into trouble otherwise.

Credit Cards
 

In hotels, car rentals and touristy places you get away with them but generally credit cards are not very popular with Greeks. Most places will not accept them.

Prices

Although Greece got quite expensive since the Euro was introduced it still is reasonable. Most of the fun stuff is still cheap while daily life products like shampoo and sun screen are dear. To be honest, as a tourist you live on the sunny side.
 
Booze and smokes are cheap, eating out is sometimes cheaper than buying the stuff in the supermarket and for a short stay that is pretty much all you need. Living in Greece is a different story. The good news is that supermarkets like LIDL and ALDI started to discover Greece as a market and if you do your shopping there its OK. Another good place to buy are markets. I wouldn't buy my Armani there but if you are OK with cheap stuff that last for only so long these for example flea markets are just fine.

Emergency: Western Union Money Transfer
 

Western Union works in Greece just as well as it does in the rest of the world. It is expensive but quick. Most banks, travel agencies and post offices offer the service.