Hi Intervoice,
You can check out ferry schedules on the following sites:
http://www.gtp.gr/
http://www.greekislandhopping.com/
Bear in mind that the summer timetables won't have been finalised yet - you need to keep checking up till the last minute. "Greek Island Hopping", by Frewin Poffley, is a very useful book, but remember that schedules will be based on the previous year's. At Piraeus, ferries are grouped together according to destination - I think there is a map in the book. You may need to consider pre-booking tickets if you're travelling in peak season, particularly around 15th August, the festival of Panagia, when many Greek people travel to the islands.
I agree with the previous posts about the Athens stay. You would be better leaving that till the end of your holiday as ferries can be cancelled at the last minute, a bit of a bummer if you're stuck on a remote island with a flight to catch. You could make your way back via one of the islands close to Athens (e.g. Agistri or Aegina) where boats are less likely to be affected by strikes or bad weather and there are so many ferry options you can leave it till the last minute before heading back to the airport - we always spend a couple of days on Agistri before flying home.
Hope this helps.
Regards,
Heather