Well ...
Athens = Hot and polluted, don't go there now, better check another time...
Crete = Nice beaches and sights, but the owners of tourist shops try to steal you, everything is so expensive
Thessalonikh = Less polluted than Athens ,but better check an area with more beaches :P
Probably choose one of the small greek islands like Paros, Skiathos, Naksos cause they are better for your holiday