October is a good time to go to Athens. There are less tourists and the weather is good for sightseeing.
The islands, however, are not in their prime in Oct. They will still look like the most beautiful places you have ever been to, but it's too cool to hit the beach And, if you want parties, there won't be any. You can get good deals on whatever hotels are open, but the reduction of other services, such as the ferry schedule, are too much hassle to deal with.
I haven't been to Crete, but heard that it remains very inviting during winter. However, their architecture is not exactly the "white-washed" theme that you desire.
I was in Athens for 1.5 days last summer, and it was enough. I saw everything I wanted to see, and got out. Athens have some of the greatest historic sites in the world, but it all seems grey and monotonous after coming back from the magnificent islands.
