We "accidently" did what you suggest.. Got off at the Monastiriki station... You end up on the wrong side of the acropolis and basically have to walk all the way around it to get to an entrance (which is the one by the Acropoli station. I would not do this. We did end up going to see the site you are talking about. If you go to the Acropolis first (do this because if you don't beat the tours form the ship it gets very crowded) then you can go back to the Metro and get off at the Syntagma station and that will put you right at the Parliament building and its an easy walk to the temple of Zeus and from there to the Plaka area. We did all of this and had lunch in the Plaka area and got back to the ship with plenty of time.
Dave... I'll bet that wasn't the first nor the last time you were on the receiving end of a kick to the backside
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