Once you are in Piraeus/Athens it won't be hard to find a taxi driver to go with you all day long to all Athens' main tourist places.
When we did this in 1997 we paid about 60 Euro's (ok it were Greek drachma's in those days) for 4 people all day long.
I think it will be a bit more expensive nowadays, but it won't be hard to find any taxi driver to do this kind of trips with you, and much cheaper then the ship tours.
Sailed on :
La Palma, Amerikanis, Southern Cross, Azur (2),
Bolero, Calypso (2), Carnival Victory
Mi name is Norma, and I was in Athens few months ago... Better than take a "taxo tour", I strongly recommend you to walk the city... the down town is the most important part of Athens... you can visit the very famous places like Parthenon, Plaka, Monosteraki, Sintagma Square, Kolonaki... you will enjoy greek mood !!!