Nice : D
First, usually is very hot in this period so have a bottle of water around you is always good and If you are coming from a Northern country make sure that you have sun protection with you.

Your first walk should be at Acropolis hill. After you have spend some hours there go to the Museum of Acropolis, it doesn't take long. If you have student card make good use of it.

Secondly, places to walk around by night is Thissio, Plaka, Monastiraki and Psirri which are next to each other and have different bars. You may have beer for a fair price on a small rooftop of a hostel called Athens Style at Psirri, where they hang around Greeks and foreigners.

Last, you may take the tram and go to Flisvos Marina and walk along side the Aegean sea. Make it there before the sunset.

This is the top 3 for me if you visit Athens for a few nights. Ask where you should eat.

Looking for hearing your experience!