Sometimes you may find yourself with a long layover at the Athens airport, or traveling from Athens to an island and wanting to lighten up your load. Two good left luggage options in Athens, one downtown and one at the airport can be found here:

Pacific Ltd / Pacific Travel Agency (airport luggage storage / Athens International Airport Left Luggage
Rates as of 2011, Contact Pacific for Current Rates

Pay when you return and pick up your left luggage.

SPECIAL ITEMS (i.e. bicycles, surfboards etc) WILL BE ACCEPTED ONLY IF THERE IS SPACE AVAILABLE
The charge for SPECIAL ITEMS is 15€ per piece per day
Minimum charge of 30 euro required for credit card charges.

Monasteraki Metro Station Locker Information

You can also store luggage in the large lockers at the Monastiraki Metro Station. This is handy if you are traveling between the port and the airport, and want to spend a few hours or the day exploring the historical center. Do realize, however, that there are only a limited number of lockers and you will be restricted by the size as to how large a suitcase or bag you can store.