Spring should start somewhere in March. Early Oct could be still mid season, after which will be low season until end May. I found this paragraph on the web, you may want to think about whether you still want to travel during this period of time.
"The low season, from October through May, generally has cheaper airfares, lodging, and food prices, but many sights and accommodations have shorter hours or close altogether. At this time of year Greece hibernates, resting from summertime farming, fishing, and tourism. Ferries, buses, and trains run considerably less frequently, and life is quieter."
I will be going to Athens and Santorini in end May, early June 2009, I remembered the weather was pretty hot when I was in Athens and Mykonos in June 1999. My sis went to Santorini in end Sep 2000, both early June and end Sep are mid season, the price is slightly cheaper and sights are not as crowded. I am not sure about the situation from Oct to May.
For itinerary, you can actually search from the internet, type "greece sample itinerary" or if you want to be more specific, search "athens sample itinerary", should be able to find some.
Hope the above helps.