Unfortunately, you don't really have much time at all! You must be IN LINE 2 hrs before take-off = 13:50, and it takes at least an hour to get to the airport from Central Athens maybe 70 minutes from PIreaus; that makes it 12:30 you have to start for the airport. What's left? 2.5 hours, IF (and I stress IF) you can get off the boat at 10 sharp. ... PLUS, you will have a bunch of luggage (in my experience, people doing cruises tend to pack heavy, lots of shoes, dressy stuff etc). You will already have been trotted around to the major ancient landmarks of Central Athens on previous day... at least that's done.
Maps can help to envision options. This map http://www.greeceathensaegeaninfo.com/a-ath/maps/07-greater-athens3-lg.jpg shows Greater Athens, Piraeus and the Airport. The main port area has insane traffic, rather scruffy seafront street to say the least, but there are nicer parts. Here's a map of the whole port -- http://www.mappery.com/maps/Pireas-Hotels-Map.jpg Your cruise dock is about where the Red K is. YOu can see a stop for the Express Bus to the airport ( E96 -- bus is now called X 96). It takes about 65 minutes, costs €6 (half-price for over 65). But don't go there, at least yet.... see those Roundish coves to the right? The smaller one -- Mikrolimano -- is a "Yacht Basin" and quite upscale. My Greek friends say the best seafood restaurants are there, and lovely views of luxury vessels. You can taxi over there, look for a nice cafe with an attractive terrace with comfy chairs , stash your luggage with the hostess, then sit & relax (it will be the last comfortable seat you'll have in a while!!) treat yourself to cold shrimp & a cold goblet of dry white and then a little before 1 pm, taxi from there to the airport. Good place for final photo op.