The two of us would like to find inexpensive ground transportation, e.g., a shuttle bus or train,from Piraeus port to Athens and return. We only have the one day in port so convenience is a factor.
For all three of your queries, go to www.cruisecritic.com and click onto "ports" on the orange band at the top of the screen.
There is a metro line from the port to downtown Athens. It is right at the harbor near the ferry station. Depending on where you ships docks it could be a walk of a mile or so to the station. Just walk along the street following the harbor.
You may be travelling on the same cruise route we did....It costs at least 25 Euro for a taxi downtown. We walked to the port entrance which is about 1 kilometer and caught the bus down to the Syntagma square which is the main centre and where you can see the changing of the guard outside the Parliament building.You can pick up another bus or Metro anywhere in the square. E mail me for any further assist.