Greece is wonderful.  Go to an island, but DON'T go to an island that has an airport.  Too touristy.
Sifnos is terrific--we spent 3 weeks there in September and loved every day of it.
On the mainland, Nauphlion is wonderful, and the train from there to Olympia is breathtaking.
If you do Athens, don't miss the national museum.

Greece is awesome! If you go to Athens there's no way you'll want to drive anyway. It's just too chaotic. And besides, it's way too much fun to walk on those ancient marble streets. And there are ruins pretty much everywere. Public transportation is quick, cheap, and (in Athens anyway) amazingly clean. If you can get to Delphi, go for it. And eat lots of that delicious Greek yogurt drizzled with their rich, dark honey. As you can see, I yearn to go back . . .

OK,Greece is great -- that one can learn from friends, any number of mags and so on. Umbra's contribution is far rarer info: how to do it "ecologically."

As Umbra noted,...you should buy green tags to compensate for your mileage...Alternatively, you could simply donate what you judge to be the correct amount of cash to the environmental group of your choice.

I'd be most interested in the specifics of the choice made, or in retrospect should have been made, in particular, what was/would be the full cost of the fare?  

In the long-ago, I got across the Atlantic via sea-travel.  Student ships, a freighter, tourist class on the Queen Mary I -- can one utilize such modes still with a better deal on the "green tag"? First-class on a ship means dragging a 5-star hotel around, so the "green tag" would show a huge $-value, I assume, but a SLOW boat with simple accomodations, might it fare well?

Greek islands are a wonderful way to vacation green. Aside from ferry travel between the islands all the amenities are in walking distance or there are shuttles that take you to beach, shopping or what have you. I never used a car during my 2 week vacation.

Mykonos has a wonderful, organic bakery in an ancient stone building.

Places like Santorini are entirely dependent on foot traffic with outrageously beautiful views down winding paths.

Hydra is car-free, with all commerce and travel conducted by donkey.

And as a vegetarian I couldn't get enough of the  DELICIOUS, locally produced dishes. So yummy. If you eat fish you'll have even more local choices.