Santorini or Meteora

Flying, driving by car or bus travel time ranges from 4+ hours to just over 6 by bus to get to Meteora.
Flying to Santorini from Athens takes 2 1/2 hours, car ferry 6 hours and combination car ferry and ferry is 8 + hours.
Check ROME2RIO for the above info.
I’d fly from Athens to Santorini the day you arrive.
I loved Meteora but I would not do it unless you had another day.

I think both options are too far for a day trip if arriving jet lagged. There’s enough to keep you occupied in Athens.

I would stay in Athens, unless you already have a couple of days tour with your cruise. There is a lot to see there. We spent 4 nights in Athens and didn't have any shortage of things to see and do. We flew to Santorini when we first arrived and spent 2 nights, but as others have said for one night, it wouldn't be worth the time. Meterora is also a long trip. They do have all day bus trips and I think train, also. But if you are just flying in from the US, you will be too jet lagged.

You don't have enough time for either of those two trips. Does the cruise not go to Santorini? I think most tour company local bus trips to Meteora are two nights-there's a lot of overhead time involved, like the ride, and crowding to enter the attraction.

If sea conditions prevent a stop at (for example) Mykonos, you might go to Santorini as a substitute. Have you been to Athens already? Does the first day on the ship include a ship-excursion bus trip to the Acropolis? How could you leave that out? A more appropriate excursion, if you don't care to see Athens, might be Epidaurus or Delphi or Napflio. But there is no need for a trip with such a short stay in Athens.

Clearly you plan to do the pre-embarkation trip as independent, which takes care of what happens if your plane is late or there are Covid-testing problems on entry. But an organized trip is a far more efficient way to deal with limited time. I'm a big fan of independent travel, but you have to recognize reality.

I concur with everybody--there is too little time for either Meteora or Santorini. Less is often more when one is traveling.

One problem not mentioned, while not a regular occurrence, it is not that uncommon for ferries and even flights from the islands to be delayed due to wind or storms, something you can not afford.

I too would just recommend staying in Athens. If a day trip is a must, I would look to something closer to Athens from a tour company, perhaps Delphi, maybe Mycenae/Epidaurus.

Getting to Meteora is way too time consuming, and I believe there's only one train each day at 07:20 followed by a taxi and a hike into the monasteries. The absolute best use of your short window of free time would be to fly to Santorini, preferably on a flight you can book for the same day as your arrival on the 9th, so as to have the most time possible to enjoy one of the most unique islands in the world.

I agree that you cannot do either of those as a day trip. I am afraid you are totally underestimating the distances. Going to Santorini by ferry will take you about eight or nine hours - one way. Sure, you can try to find a return flight that will take you there and back on the 10th, but what sense does that make? You will need at least an hour to even get to the airport in Athens, plus the time you need for security etc. The flight is not that long, about an hour, but altogether you can safely assume a three hour trip each way. Then, you will arrive at the most unattractive spot on the island, and you will need to get a rental car to even enjoy those few hours before you have to be back at the airport. Is that worth it?

You have one full day only, plus - depending on your arrival time - two half days in Athens. That is barely enough to explore the main sights of Athens. Stay there and enjoy your valuable vacation days seeing amazing sights and not only the road.

No way to do those

Having been to both, I'd say Meteora is ABSOLUTLEY the one to do! We took the train to Meteora and stayed two nights before taking the bus to Thessaloniki. The train is easy to do, but it's also very drivable. The roads are pretty nice and traffic signs easy to read. We took the half-day sightseeing AND sunset tours from Visit Metorea, which were fantastic! The tours go to different monasteries and know the scenic stops for photos. The drivers can easily navigate the twisting roads and multitude of cars. This really was one of the best places we have ever visited!
https://www.visitmeteora.travel/package/meteora-half-day-sightseeing-tour/