We're going to have a layover in Athens for eight hours during the day beginning of January and want to go visit the city. Our checked luggage will be moved to the connecting flight, but our cabin luggage is going to be a burden to carry around all day.
Are there luggage lockers or similar facilities somewhere in the airport? Is there an alternative like taking a commuter railway to the central station and locking it there?
Yes, there are facilities there. Core4Bag provides luggage storage services at the airport. Price depends on the size of the bag and how long you're storing it there for.
Here's the link to the airport luggage services page, which lists them, along with their phone number: https://www.aia.gr/traveler/airport-information/baggage-services
And here's the link to the Core4Bag web site, which among other things, has pricing information and directions on how to find them in the airport: http://www.care4bag.gr/
From their website:
Baggage storage is located at Arrivals Hall next to Gate 1. Operates 24 hours. All stored items are controlled by X- Ray and monitored by CCTV during all storage time.
Here's the direct link to the pricing table: http://www.care4bag.gr/baggage_storage.html - if you're talking about an average cabin carry-on, it's going to be either "small" or "medium" in their terminology, and will, respectively, cost €3.50 or €5.50 to store (per bag) for up to 6 hours. Note that their pricing table states that these prices are valid up to 31/12/2015, so may change from January.
