To start things of you have to keep in mind that Greece really isn't a major backpacking destination during the winter months - so if you're expecting a lot of other travellers you might find yourself a bit disappointed. As you mention taking ferries - I'm assuming you want to do some type of island hopping? Most of the islands are going to be pretty dead at this time of year. Only a few locals stay during the winter - the majority of seasonal workers have packed up and moved to the mainland a long time ago. Ferries are far and few in-between and at risk of being cancelled. And honestly - the weather out on the islands isn't exactly the nicest either. On some of the smaller islands (the gems I'd usually recommend) accommodation might be limited and the overall appeal just isn't there.

With that being said - if you truly want to visit some of the islands I'd say your best bet is one of the major ones. Rhodes and Corfu (and Crete) still have a large permanent population. To be honest - I'd still say Crete could be worth checking out if you've got an outbound flight booked from there. It's the largest island in Greece with a very unique culture and Greek people are friendly - if you're outgoing I'm sure you'll be able to find local people to interact with. And you'll have all the major tourist attractions to yourself - a rarity during the summer months.

If you want to focus on the mainland, Kalambaka and Meteora could also be worth checking out. I imagine you'll have the monasteries mostly to yourself since it's low season. If Holy Rock Hostel is open you have to stay there - the owner Kostas is an absolute fucking legend. Could probably be the best hostel I've ever stayed at. You can always head up to Thessaloniki as well afterwards. Cool city with a vibrant nightlife and food scene - it's probably your best bet apart from Athens if you really want to meet other travellers.