Greece or Spain/Portugal in December (10-12 days)

Although it is impossible to predict what the weather will be like in more than 3 days, the climate and average temperatures in December of these cities are roughly the same, roughly between 10 °C and 15 °C.

A weather that does not encourage staying on the beach.

You will find history and architecture everywhere in Europe, from thousands of years BC to contemporary history.

Same for good food.

Just be aware that travel between Madrid and Lisbon requires a flight. For such a short time why not add another city in Spain such as Seville. You won’t be disappointed.
At the NY Travel show in 2024 Rick mentioned a high speed train between Lisbon and Madrid being built but I never read anything more.

I'm with a response above, for a group, getting from Madrid to Lisbon is a pain, so concentrate on Spain, adding Southern Spain to the mix. How long a trip are you talking about? Will this include the days over Christmas? Those two answers will heavily influence any other advice. The South Coast of Spain will be in the low 60's, maybe fine for sitting on the beach, but probably not swimming much. Madrid will be cooler, low 50's.

Some Comments about Greece: December is fine to visit Greece, but not really the islands. Many places on Mykonos will be closed, Santorini seems to be year round now, but still, places will be closed. Athens, and the Peloponnese might be pleasant though.

I can give you the view of someone who is Portuguese but knows several cities in Spain and some in Greece, particularly Athens.
As they are all southern European cities, you'll find a relatively similar lifestyle. I don't think the weather in December will allow you to go to the beach in either country. Maybe a stroll along the seafront...

I can tell you a bit about Lisbon and Portugal in December, usually you'll have a pleasant temperature and far fewer tourists than from March to October.

Portugal is a much smaller country than Spain or Greece, so you'll be able to see many sights within a short distance of Lisbon or Porto. Renting a car can give you greater freedom and in 5 or 6 hours you'll be in Madrid if you want to.

If you're interested in finding out more and need help planning your trip to Portugal, I'm more than happy to help.

10-12 days is not a lot of time.

I have been to all three countries. I would not go to Greece in December if your interest is in the islands. Very little is open.

That leaves Spain and Portugal. I would pick one. You could add Toledo and/or Cordoba to your Spanish itinerary. Or Sevilla. The weather will be nicer in the south in December than further north.

Porto is lovely but perhaps not as warm as you would like. You might lalso ook at places like Evora or even the Algarve in addition to Lisbon. We were in the Algarve in mid October and really enjoyed it. If you go to Lisbon, I would certainly go to Sintra as a day trip.

I wouldn’t recommend visiting the Greek islands in December, as most businesses shut down for the off-season, and it tends to be windy and cold—making it difficult to enjoy the beach. Instead, you could explore Athens and take day trips to mainland destinations such as Nafplio, a seaside town with Venetian architecture, Sounio, home to the stunning Temple of Poseidon, and Delphi, one of the most important archaeological sites in Greece.