As a native Greek living in Athens, I assure you that October is a perfect month to visit Greece, especially if you can come around 8-10 of October.

There are still tourists but the places are not overcrowned, you can find very good hotels in nice prices and you can see the wonderful automn colors in the trees...

Either mainland and islands are good choices. I have been twice to Rhodes island at the end of October and I loved it. It was better than August.

Best Regards from sunny and warm Athens, Joanna