Well December is a relative cold month.
if i had to go on an island on December, I would go for a big island and south. Best choice here is Crete.
Another option is to see how the weather is and if sunny and relative warm, you can always hope to Aegina, Agkistri, Hydra, Spetses or Poros. All of them are about 2 hours by boat away from Pireaus (the port of Athens). about 6 years ago
Crete and Rhodes are the best options. Cyprus has a similar climate to both these islands and is also beautiful. If you want to combine sights, archeological places and nice towns and villages, these would be the options. Of course they have beautiful beaches also, but maybe you can visit again summertime. about 6 years ago
My recommendation is to rent a sailing boat and go to see some great islands in South of Aegean (best time ideal prices!) and also in the south coast of Turkey
Otherwise Peloponnese is always a classic choise :-)
grazytravel.com about 6 years ago