I would stick to the mainland. Winds can be high in December and ferries delayed. You could fly to, for example, Crete but you then lose a lot of time travelling to airports and sitting in departure lounges.
One of the best options, and a favourite of many of us here, is Nafplio. It’s around two and a half hours by bus from Athens and buses are frequent throughout the day. The centre of the town is pedestrianised, it has a great walk round the headland and a massive Venetian castle. There’s a very good range of restaurants and bars.
A lot of good information here
http://www.visitnafplio.com/
Have a great trip. I really like Greece in December when everywhere is quieter.
Alan