Ferries operating to Santorini from Dec 20 onwards?

There are ferries all year round, residents are the first to use them, and the islands must be supplied

Piraeus ↔ Santorini is a ferry route operating 365/365 with Blue Star Ferries and Zante Ferries
However, be aware that the journey to Santorini lasts 6 to 12 hours depending on the ferry and its intermediate stops.

There are no Seajets Fast Catamarans in December.

There are several ferries per week to Hydra

Whether in Santorini, Hydra or on any island, there will always be some accommodation and restaurants open but don't expect a hectic atmosphere. It's an island version of daily village life during winter.
Is it worth going there or not? It depends 100% on you and your way of traveling.

Also note that the weather is unpredictable and can lead to ferry cancellations for several days. If you go to Santorini take a flight with Sky Express or Aegean / Olympic, it will be quicker and safer than a ferry and won't be much more expensive.

Consider the weather -- and the fact that many tourist-heavy islands will be fairly "shut down" in mid-winter ... and Hydra is no exception. If you have a sunny day, low wind, perhaps you might consider a 1 hour ferry ride from Piraeus to AEGINA ... this isle is close enogh that many Greeks commute from there to work in Athens. Be sure to go on a Conventional open-deck ferry, NOT a hydrofoil (closed-in, no view). Check weather forecast & buy ticket at an agency close to your hotel so can choose right ferry (there's usually one about 8:30 am); have hotel get you taxi (from €18-25?) to Ferry Gate. Here's a great photo-sequence of Aegina highlights in June - http://www.flickr.com/photos/36264706@N03/sets/72157621604646139/detail/ I don't know about staying o'night, but if you do, find a hotel in the port, where most activity is, for locals -- restaurants,shopping, night life.

Another excellent trip for at least 2 nights can be NAFPLIO on mainland ... just 2.5 hours from Athens on highway (via rental care or comfortable modern intercity bus). It's at tip of a peninsula, surrounded by the sea, so feels like an island. Its considered THE most beautiful Old Town in all Greece - gorgeous golden Venetian architecture, with huge Gibraltar-like hill fortress of Palamidi looming behind it. This superb website -- http://www.visitnafplio.com/ -- tells all; whre to stay-eat-explore etc; cllick every link! Here are photo highlights by top RS contributor Stanbr: https://www.flickr.com/photos/visitnafplio/sets/72157626483407223/ However, Nafplio is vibrant year-round, culture & nightlife, nearby university .. here's lively video in February. https://vimeo.com/9186703 Nafplio has seafront row of cafes with see-through side-panels, so even in chilly weather you can enjoy sunsets (better than Santorini's, IMHO). Going off-season, you might score a seafront room at Hotel Agammemnon, best (moderate) hotel in Old Town. you'll love it!

Thank you so much for feedback from the both of you. Really helpful. It appears that it may not be advisable to plan a trip to faraway islands like Santorini, even by plane since the weather is unpredictable and more rainy at that time.

It was good to know that the ferry ride would be 6-12 hrs. Definitely not what my husband and I can tolerate.

Aegina looks like a great possibility. Will plan that into our itinerary.

We do plan to visit Nafplio. This is our current but very rough itinerary: 3-4 nights in Athens (including a day trip to Aegina), 1 night in Meteora, 1 night in Delphi, then on to Corinth, Nemea, Mycenae, Napfion (2 nights), Epidaurus. I haven't determined how long to stay yet in some places or what else to include since it's likely we won't go to Santorini.

Since it's such a long drive to Meteora, are there other attractions in that area that would be good to visit?

Also, since we will be there in Greece over Christmas, I gather that it would be best to be in Athens on Christmas Day itself to ensure that we can find restaurants to eat at. Will any attractions be open at all on Christmas Day?

I think Meteora is not the best idea in winter ... long drive in iffy conditions, better for skiing, not climbing up pointy rocks... will take you much longer in travel than you think ... you will spend too much of your Greece visit in the car, staring out at the rain (snow). Don't be greedy ... save it for your next trip.

Oh ok... I really do need to rethink my whole itinerary. Any suggestions then taking into account the weather? My husband and I do need to come back to Greece at another time.

I have been reading through previous posts to get an idea of other places I should consider, it appears I should focus on the Peloponnese - Messene, Mani Region, Monemvasia. Agree?

I also think the itinerary you are planning is too busy. Like many others you want to go in opposite and incompatible directions in such a short time, and you add the winter period. This means too much time spent in the car driving in unfamiliar mountainous environments with rain, cold, fog, nightfall at 6 p.m.

Without realizing it, you are transforming what is supposed to be a vacation into an obstacle course.

Start with 3 nights (2 full days) in Nafplio.

Then if you are heading towards Messene or Magne you will also have to take into account the weather and winter driving conditions in a mountain environment. Also note that the sites close around 4/5 p.m. in winter. Some like Mycenae at 3 p.m.

You will quickly understand once there that you will run out of time.

As for Christmas in Athens, here are some tips:

https://www.thisisathens.org/seasonal-guide/winter/athens-christmas-like-local

https://www.thisisathens.org/seasonal-guide/winter/christmas-holidays

Thank you so much for all your valuable insights. Really really grateful for this forum. It helps me take many important things to take into account.

I have been revising my itinerary and this is what I have come up with:
Arrive on 19th Dec
4 nights in Athens. Include a day trip to Aegina.
3 nights in Nafpolio. Take in Epidaurus, Mycenae, Argolis, & Tyrins during that period
(I read it's fine to spend Christmas in Nafpolio)
Visit Messene and Kardamyli. Spend the night in Kardamyli.
Visit Nemea. Spend the night.
Visit Corinth. Spend the night.
Drive back to Athens. Stay near the airport.
Fly out the next day on Dec 30. This departure date is flexible. We could stay a night more as we have not booked our flights yet. We are trying to finalize the duration of our trip beforehand.

If you want to visit the archaeological site of ancient Messene, note that it closes at 3:30 p.m. in winter. So don't leave Nafplio too late in the morning.
The last kilometers to get there are on small winding mountain roads. But the site is worth it.
Check the weather forecast in the area (Kalamata).

https://www.meteo.gr/cf-en.cfm?city_id=15.

Still if you visit Messene, after leaving the site you can take a lunch break at "To tiganaki tis Xristitsas". This is a traditional Grill house in the village of Arsinoi, 10 minutes' drive from the site on the way to Kardamyli. I checked, it should be open late December.

To tiganaki tis Xristitsas:
https://maps.app.goo.gl/J1DVqF3seE9tguDUA

Be careful, the road to Kardamily is very winding and mountainous after Kalamata. You will not drive faster than 25 mph.
And if you want to go to Nemea after Kardamyli you will have to take this same road in the opposite direction and go back through Kalamata.

If it's open in December, a place to spend the night in Kardamily: Ifigenia's Rooms:

https://maps.app.goo.gl/D4vQxg6B1pfMuhRm9

To eat, go see if Giorgitsa’s Backyard is open, but I have no idea if there are tables inside :))

https://maps.app.goo.gl/2p9noXNdBKdh4y7fA

It would be much less boring to spend the last night in Athens instead of next to the airport.

Wow, thank you again for incredibly valuable info. Super super helpful. That really helps my husband and I.

We will definitely visit Greece in warmer weather.

Many many thanks!