You are choosing a great time to visit greece as far as the
weather, but because August is the height of high season, you will need
reservations. I don't recommend trying to book accomodation or tours after
you arrive. Also, with flights, bear in mind that you won't be able to fly
island to island (with a few exceptions, you can fly from crete to Rhodes,
and make one or two other connections in high season). You would need to
return to Athens and then go to each island.
Also remember that Athens is hot and noisy and dirty--you may want
to chop a day or two off and add it to the islands. One island that I
recommend is Paros (we just got back from 2 months there) despite the
unfortunate ferry tragedy.
Halifax, Nova Scotia, Canada