You could easily fill it with the acropolis and museum on 1 day, then the oracle at delphi, then corinth canal, and then a day to just wander athens and recover.
If you've had a busy cruise then you might not want to go as far as corinth and delphi as they're full day tours, albeit not very strenuous coach journeys.
Recommend the Athens Gate Hotel if you do stay - fantastically located and the restaurant has the best views of the acropolis at night.