You will easily be able to see most of the major attractions in Athens in 2 days.

Definitely go and wander around the Plaka and Parthenon as suggested above. The Roman Agora can also be found in this area and is a great place to chill out.

However, if you do get the time, then it would definitely be worth taking a trip to Delphi (home of the Oracle) for the day. It takes a full day to travel there and back by coach, but if you get a good guide then the journey will fly past and the site is absolutely amazing. It was definitely the highlight of my stay in Athens.