Just a heads up that the metro and bus tickets have gone up again, certainly since I was last there in May.
Single metro tickets from the airport are now 10 euros and the airport bus is 6.
The standard ticket is now 1.40.
Full details here
http://www.athenstransport.com/english/tickets/
Oh, and in 2017 Athens is going over to a travel card system, which should prove interesting.
Alan