I don't think you have time for much sightseeing, except from a window of the car. You don't give exact Date -- but if you arrive on a Friday or Saturday, mid-morning or later... you could encounter another BIG plane landing at same time, resulting in long Passport Control line. Then waiting at Baggage Claim, getting Euros from ATM .. it could be over an hour... when someone is connecting with a domestic flight we usually caution to allow 2 hours.
So... now y ou're out the front door .. A taxi or Car Service from AIrport to the Cruise Gates could take over an hour, because Cruise port is farther than ferry gates. (check the routes & roads looking at this map -- https://tinyurl.com/zekuft9 --click & it gets huuuge... use side-slides to move to upper Right for Airport - athens - Pireaus) As u can see, if you want to see landmarks in Central Athens, you would get into the same kind of traffic you'd encounter in any major city ... and an added handicap is that major landmarks like the Acropolis, etc are surrounded by "no-cars" small streets, so you can't get very close. It could be frustrating.
Here's a thought -- do you want to see something Gorgeous and not surrounded by busses &tourists?? Look on the map -- At the very tip of the Peninsula leading down from Athens is the TEMPLE Of POSEIDON ... a beautiful ruin on a cliff. At sunset it will by ringed by tour busses, but if you go there at lunch-time, it won't be crowded. https://www.greeka.com/attica/athens/sightseeing/sounio-poseidon-temple/
I would suggest writing to the best car-service currently, Welcome Pick-Ups (drivers are fluent in English, you know the cost beforehand & pay by credit card). You can find out what it would cost to drive you to Poseidon... wait while you see the site, (and maybe wait while you enjoy a snack & beverages at the cafe), then take you to Piraeus. (An optional add-on would be get a quote foro also taking you to a lunch place & wait while you lunch ...