Agree with above. You can see all there is to see in Athens in 3 days, then you can spend the other 4 in one of the nearby islands, 1hr away by hydrofoil. Haven't been to Hydra, but Poros is another great choice (we just came back!), stay there a few days, rent a scooter, explore the island and its beautiful beaches, and get to know the locals, you will have a great time.

BTW flew Olympic on CDG-ATH and the service was pretty good and fairly comfortable seats (for coach!). But you will most likely get smaller planes to the islands...