A valid US drivers license is all you need to rent a car in Greece. We have rented many times in Greece with no problems. Technically, you should have an International Drivers Permit (available at any AAA office) although nobody will ask for that when you rent a car. In the event that you would get stopped by a cop it is possible (but not likely) that he/she would want to see that International Drivers Permit. As I said, they are available at any AAA and cost $15. You can certainly hire a taxi for the day if you find a taxi.....and there is no guarantee that a taxi driver will speak much English. Of course the cost would be negotiable and can run into the hundreds of Euros.