The Greek currency is the drachma. You can easily find many places on the island, aside from the banks, that will change money. In many business establishments (souvenir shops, etc) you can pay by credit card, although cash is always preferred. Exchange rates, even in small outlet stores, are updated daily.
The Banks operate weekdays and they close at 2 pm (1.30 pm on Fridays). Automated Teller Machines are proliferating all over Greece and there are a few on the island.