Welcome to Cruise Chat dv0823!
The Plaka area after the Acropolis is a nice combination. You could do more but if you want to be able to enjoy a cafe or 2 and do some shopping then I would not attempt to do much more. If you plan on taking the Metro (subway) to and from the port, then there are some nice archeological ruins sites between the Plaka and the station you could visit if you have time on your way back to the ship...
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007