I visited Athens and Delphi in December for a few days, which was enough time to check out all of the famous historical / archaeological sites, and enjoy some amazing Greek food!
Kerameikos & Kerameikos Museum
Approx 1.hour to see everything.
Ancient Athenian Cemetery, within the Agora.
A trip on the metro to Piraeus is worth it!
This took up most of our afternoon, including travel, lunch, and a wander around. We travelled from Thissio metro station (Metro Line 1 / Green) to Piraeus, then the museum was approx. 30 minutes walking.
The Piraeus Archaeological Museum is great!
National Archaeological Museum
2 hours. Metro line 2 / red, north from Acropolis Museum to Omonia, then a short walk. Oktoyriou 44, 106 82.
A wonderful museum with a lot of ancient sculpture to see!
Afternoon exploring / shopping / eating Greek food!
As I went on a university trip, we had a lecture planned for that evening, but you could use this time to check out anything you'd like, or get some ouzo at a bar!