Kefalonia, yes, yes,yes! Beautiful scenery, lovely people, nice beaches, (Mounda near Skala is gorgeous!), great food.

YEs Kefalonia is my favorite and will be returning on the 26th Sept got a very good deal flight only with My Travel do not know what you want S/C hotel or R/O, Corfu has some nice places but October there can be the rainy season Don't know how long you are going for. Lesvos has some lovely beach also Skiathos more beaches than anything else really I'm not a beach person personnally but must admit SKiathos has had a lot of really sandy and well kept beaches I have seen and have travelled many Greek Island, But Kefalonia has always come out tops.
pojo

Georgioupolis, Crete. 10km of sand, small village, lots of tavernas and bars. Beautiful scenery both coastal and inland including Kournas a freshwater lake. Samarian Gorge near by. Chania and Rethmnyon close by. Lots and lots of historical sites such as Knossos (about 2 hours by car) Phaistos, Gortys .

Kefalonia is great but if you want some night life there is not a great deal - it is a big island and therefore you tend to spend your evenings wherever you are staying. Skala has bars but it is very small and does not a lot of choice of nightlife, but it is a lovely resort with a good beach and watersports.
Skiathos would be my choice. It has brilliant beaches, is very pretty, small (you can drive round in less than a day) and Skiathos Town has a very buzzy, cosmopolitan, and lively atmosphere. Plenty of bars, a few clubs but nothing 18-30-ish. There is not a lot of culture though or places to visit being such a small island but hire a jeep and have fun exploring the mountains and beaches inaccessable without one. Fantastic island.

Hi Zanders, if you want almost guaranteed sunshine and warmth in Greece at this time of year you need to be looking south and east at the Dodecanese... Rhodes, Kos, Halki, Symi etc.
Have a look at Kos... stay in Tigaki or Mastihari for the beach and visit Kos Town for the architecture and culture.
Rhodes is good too, Rhodes Town has lots of history and architecture and at this time of year the fantastic beach at Faliraki won't be heaving with looney Brits!
Kefalonia and the Ionian Islands are lovely (my favourite actually!) but at this time of year the weather might not be so good...
Let us know what you decide on !