I was there in Dec of '03.
Approximately 14 km. N of Athens, there are ruins of Elufsis, and the entrance to Hades is supposedly there.
Delphi, several hours North of Athens is one of the most spectacular places you can go if you want to take a cab into the mountains. There is a theater, a huge temple, a stadium (I actually ran around the stadium, and was breathing hard, the altitude is such). It's also the basis of an Indiana Jones book.
Cape Sounion is a great place as South as you can go in Greece. Maybe an hour or something south of Athens. Has Temple of Poseidion, and if you're there in Jan, (As I was there in Dec) the wind just might blow you off of the steep, steep, steep incline leading to the sea, next to the temple, and then you have a real Indiana Jones style adventure.