Thessaloniki. Take it! End of September, it will still be warm, but not scorching. See the Thessaloniki forum for things to do. To reach some interesting sites, you will need a car - it's worth it. You can get in from
Thessaloniki Car Rental.
If you have done Athens before, keep the car and drive down to Nafplio and stay there for a few days to take in Corinth, Epidavros, etc. It's a bit of a haul but not too bad in September.