If you are planning to go in December, as your other post suggests, best stick with Athens where there are lots of ancient sites and museums. The Argolida region around Nafplio has many things to see. Delphi is also a good ancient site. On the top of Mt Parnassus, where Delphi is situated, is one of the biggest snow skiing centres in Greece.

There are so many ancient sites, plus islands, You probably need to look at some guide books and perhaps history or art history books.

You can find the "best attractions" by reading guidebooks. Personally, I would not choose Greece in December, and if I did I would stay on the mainland.

Meteora