Take a cab.
It does not cost the world:
There is a cab stand just in front of the Cruise-Terminal:
Once you are sitting in the car they must drive you.
So as everybody want this great business be sure you can get him easily doing what you want!
But do not try to catch a cab in Athens itself:
You as passenger apply that they will take you as guest!!! It could happen that somebody is already in the cab and they try to find someone to combine the tour. But that's only in Athens itself.