The fact of the matter is that while there are quite a few nice classical island sites you can see many more in less time on the mainland.
The Greek mainland has great beaches too and sights modern, medeival and ancient to see.
Lodging is also cheaper in many cases than hot spot islands like Mykonos, Santorini, Rhodes, Cos, Corfu and Crete.
High season why wait in line for service on these popular islands when you can rule the roost and be treated like a king for less in many slightly off the trail mainland locations?
Be sure and consider the many guided tour options at your disposal or consider renting a car or a private driver.
There are many sites within a 2 or 3 hours drive of Athens giving you a value packed itinerary should you decide to visit ancient sites, beaches and battle fields.