I would like to purchase a SIM at the airport to use it for one week in March, please advice
It is my understanding that it is no longer possible (I hope I am mistaken!) to buy a SIM at the airport, but across from Syntagma Square in central Athens you won't have a problem finding what you need at a Cosmote sidewalk kiosk. The 2 most popular SIMs are from Cosmote and Vodafone. Both the Airport Express Bus X95 and the Blue Line Metro stop at Syntagma Square.