Itinerary help: mid Feb - late March/early April

You are on the right track with just transiting through Croatia given the transport logistics and the weather. I have been there four times and only once to Greece. Someone with better knowledge of Greek weather conditions will have to advise, but I can say for sure I would be able to find more to do in Greece than Croatia in February. If I were in your shoes, I would start with a good week in Athens, divert to the Peloponnese by car, explore northern Greece by car, ending with some time in Thessaloniki. Ryanair has a nonstop to ZAG, but not sure if that is seasonal. I would consider seeing more of Turkey versus the islands in March, but others may know a lot more.
How delightful it will be to explore the museums of Athens without massive crowds! You know that section of a guide book that tells you what to do in certain months? That is where I would start--I like the Bradt Guides for Greece--good luck!

ETA This is the kind of info I'd be seeking out: https://www.greektravel.com/winter/

You can easily spend 5 or 6 days in Athens and 3 or 4 days in Thessaloniki without getting bored.
You won't need a car; public transportation is efficient in both cities.

However, a car is essential if you want to explore the Greek mainland or the islands freely. There are almost no train lines, buses are operated by regional KTEL companies whose routes are sometimes difficult to navigate, and these buses don't go everywhere.
Archaeological sites close earlier in winter (around 5 or 6 p.m.).

I know it's not the best weather, and we will only be able to go to
islands that we can fly to.

You can travel the islands by ferry even in winter. The islands aren't just for tourists. The ferries are primarily intended for islanders' travel and for supplying the islands with materials and consumer goods. They operate 365 days a year, although they are less frequent in winter.

Daily life on the islands doesn't stop when the tourists leave. Children go to school, doctors provide care, construction workers build houses, fishermen fish, and believers go to church, just like where you live.

The only thing missing is a few beach clubs and ice cream vendors. :)

Am I correct in thinking mid-late February, all of March, and early
April are still off-season and shoulder-season?

Yes, it's off-season. The shoulder season begins around Greek Easter, which will be the second week of April in 2026.

Note that Greece is 70% mountainous, snow is possible in February and March, particularly in the northern regions, but also on the islands. There are also ski resorts in Greece, not just seaside resorts.

An immediate thought: Greek Easter is 12th April next year while in Croatia Easter is 5th April. It’s worth bearing in mind when thinking what will be open in early April.

I can’t advise on the balance of your holiday, I’ve never been to Croatia, but I’m fairly regularly in Greece in March and April. I’d describe it as off season rather than shoulder although Greece is increasingly a year round destination. Mid February to mid March can be quite cold and wet, although you’ll always have the odd sunny day. From mid March things begin to get sunnier and warmer.

As well as Athens I’d recommend Nafplio on the Peloponnese. It’s an easy 3 hour bus journey from Athens and has a lot to do in the town itself. You can hire a car there for a day or two and visit Mycenae and Epidavrus which are nearby.

Ferries run throughout the year so an island is possible, although the smaller ones will be closed up. I like Syros, the capital of the Cyclades, which is lively year round. Ferries can be unreliable if the wind gets up but, given you have a lot of time and are happy to be flexible, that’s not a problem if you’re prepared to wait it out for a couple of days. Schedule any island visit earlier in the holiday.

Sounds like a great trip.

Alan

Thanks for the info about ferries. I should have been more clear in saying I'd rather fly since winter/early spring seas can be rough, ferries less frequent and more likely to be cancelled due to weather.

And yes, I know people live year round on the islands and have regular jobs/lives. We live in a popular tourist place (Alaska) and I know life goes on everywhere when the tourists go home. I'm sure we'd be able to find lodging and places to eat. In fact, we'd prefer it to be very quiet and low key, it's one of the reasons to travel Feb-April.

That may have been more for my benefit since I spouted off about avoiding the islands. I am sure you made made of much stronger stuff when it comes to weather. I'm a southerner and a total wuss! But JoLui is someone I rely on a lot to learn more about Greece, so good to hear for me--I'd still put island time toward the end of the trip though. Maybe some islands might offer good flights to Istanbul to help limit the number of flights you have to insert.

No, you were fine and I wasn't bothered by what JoLui said, lots of people don't live somewhere with places only accessible by air or water (like Southeast Alaska) and wouldn't think of what he said, so I appreciated it. But wanted to clarify anyway.

Yes, island time definitely towards the end and weather dependent, too. While I don't mind if it snows I'd be fine choosing to drive the other way if it's predicted. Luckily we are experienced in driving in all sorts of weather and places worldwide

I will tell you though that many tourist oriented things do close up in the winter. The manager of the hotel we stayed at in Naxos told me he spends his winters in Athens.

Personally, I would stick to the mainland although as has been mentioned there can be snow in the mountains. We were surprised to see a ski resort in our way to Delphi.

Along the lines of what Beth says above, we just got back from Kefalonia and our tour guide, who is from the island, told us he and many others leave for Athens in the winter where they have other jobs. The islands won't be deserted but many things catering for tourists like hotels and some of the restaurants will close. You will also miss out on seasonal activities like boat trips around the islands (sightseeing trips, not ferries) which don't operate in winter. If it were me, I would stick to the mainland especially places like Athens and Thessaloniki. And I agree with not wanting to take a ferry in winter due to possible bad weather. We took a large car ferry from Thessaloniki to Skopelos two years ago about a week after a big storm and it was incredibly unpleasant- many around us were using sick bags. Of course not all ferry rides will be like that but in winter you are more likely to encounter bad weather. Good luck and kalo taxithi.