You can take a cab to the Acrpolis, tour there - you want to be early since it will be hot. Then you can take a cab to the Plaka. The subway is right there at the Plaka and very easy to put your money in the kiosk to get a ticket. Or you can just grab a cab from there back to port.

VOYAGER:
MC to FLL 11/07
MC/Dover 6/06
MARINER:
BCN / Ven 10/11
Alaska 2001|2006
Panama Canal 2003
NAV
Caribbean 11/06
Bermuda|New Eng/Canada 6/04
PG 5/05
OCEANIA
Nautica |Ist/Athens 6/07
Regatta|Baltics 6/08
Riviera 2013 Crossing -Istanbul to Miami