Piraeus is the main cruise port for Athens. It is about 15 minutes from historical Athens.
Essentially all cruises from Athens remain in the Mediterranean, though there is much variety among the itineraries.
Greek islands cruises are popular options, as are combined Greece / Italy and Greece / Italy / Spain. Each of these voyages can be found in 7 to 14 night versions, with a smaller number being 3+ weeks long.
There are very few long-haul repositioning cruises, but many of the Mediterranean exploration cruises are available in roundtrip or one-way versions.