Good advice, with just one added tip. On both the Airport Metro line, AND the "Green Line" that goes from Monastiraki station to Pireaus -- be VERY careful about pickpockets. Athens is a safe city, a record of lower petty crime than most other European capitals but the "pickers" do target those 2 Metro lines... because of all the unwary tourists who use them, and aren't used to public transit, AND also carry valuable stuff carelessly. These crooks (99% of whom are non-Greek) work in groups, sometimes cause disturbances & jostle people who are coping with big luggage or backpacks, then suddenly exit and whoops!! people realize their wallets or phones are missing.
Just be sensible: just for the metro ride (not on boats or planes) keep important cash, credit cards, passports etc in a money belt (or my favorite, a "neck pouch" that hangs down flat under shirt) OR, for women, in ZIPPED inside pocket of daybag with ZIP top, worn crosswise in front, hand on zipper. Wear backpacks on your FRONT during ride. Men, NOTHING of value in any pocket... even if u think you'd feel a hand - these guys are too good. be aware, choose seats near the exit doors, then enjoy the ride. By the above prudent measures, I've used the Metro many times, more than 50, with no incident.