Greece and what else?

From Athens (or Crete in summer) there are direct flights to more than a dozen countries in Europe.

But in my opinion, with 2 weeks you will have only seen a small part of Greece. Why don't you stay in Greece if you have extra days? Either for example to visit islands other than Crete/Naxos/Paros where everyone goes, or to go for example to the Peloponese.

I would suggest that you spend your extra time in Greece as well. On our first trip we went to Nafplio on the mainland. On our second, we went to Delphi and Meteora. All three are wonderful and different than the islands.

You also can easily spend a lot more time on Crete. We spent 11 nights, saw only the western part of the island, and still really could have used a couple days more.

All that said, there are direct flights to lots of European cities from Athens. You just have to choose.

As the earlier responses hint, there really aren't good ground-transportation options to other countries from the places you're going to be in Greece. It would a require a long day (at least partially on one or more buses) to get to Bulgaria, North Macedonia, Albania or Turkey. I, too, would highly recommend staying in Greece so you don't spend so much time on transportation. Mainland Greece is fascinating and, aside from the most famous sights in Athens, not swamped with tourists.

Add the Peloponnesean peninsula and stay in Napflion. Don’t skip this area!

How much time do you have, beyond 9 days?

As noted above, Crete can use more time than just a few days. Over 2 weeks, we visited the 2 biggest cities (Iraklio and Hania), plus the Lasithi Plateau in the middle, and towns, beaches, and ancient ruins sites in the east, plus some on the far south. Didn’t get far west. If you’ve only got “5 to 7 days,” make it 7, not 5 or 6.

As Suki says, the Peloponnese is a whole other part of Greece to see, without leaving the country. In addition to wonderful Nafplio and the sights in and near it (like Nemea, Epidauros, and Mycenae. Then there’s the fascinating walled town of Monemvasia on the southeastern edge, Aeropoli and the Mani Peninsula on the southwest corner of the Peloponnese peninsula, and Ancient Olympia (see the remains of the original Olympic stadium) in the northwest. So many amazing places, depending on how many days and nights you have available.

If you want someplace that’s not Greece, Bulgaria is next door, and is amazing, too. Their yogurt is better than Greek yogurt. They don’t use the Euro like Greece, so you’ll need to get Lev, which are simple to withdraw from an ATM.

Are you flying directly between home and Greece? If you’re connecting somewhere to and from Greece, like London, Frankfurt, or Amsterdam, you could potentially stay a few days and see some of the UK, Germany, or Netherlands.

Thank you to all of you who answered my question. Yes! We’ll stay only in Greece and use our time there. We can spend a week in Crete, a few days in Naxos, and another few days somewhere else. Before heading back home, we are planning to stay in Athens for 3 days.
We have time to plan this trip. We can’t go next year as is full of other trips and family commitments. More time to plan a trip to an awesome destination.

If you have a few days to fill, I would suggest Nafplio. It is about two hours from Athens. We rented a car from the airport and there numerous day trips nearby. Parking in town was easy. We had four nights there but even three would be worthwhile.

Personally, I would just add to Greece. It is geographically remote so unless you are going to a bordering country like Albania, Macedonia, Bulgaria or Turkey then just go further in Greece.

We have five roughly-one-month-long trips to Greece under our belts and we could easily double that without repetition.

You could add a few more islands while returning to Athens, like Syros, Sifnos, Serifos, Kithnos, Kea, Tinos, Andros which would all be convenient from Naxos / Paros.