I have just got back from kefalonia, what a place. This island is mostly unspoilt and the people are not yet sick of visitors.
We stayed in Katelios which is where the loggerhead turtles come and lay their eggs. Lots of students on gap years staying there patrolling the beach through the night and making sure people dont put umbrella poles in the eggs.
We hired a car which was much easier than I thought it would be, having spent many years in Greece and saying I would never drive there, it was very easy as there is very little traffic on the roads. My only advice would be that if you do decide to hire a car and go up the mountains, (its impossible not to) then be careful. There are many places where there is no barrier with huge cliffs down to the sea and no white lines on the side of the road.
Hiring a boat is a must, they have everything and you are even given a mobile phone. There are so many beaches that you can only get to by boat.
There is also the sister island of Ithica made famous by Homer, to visit as well as Zante visible from the island.
Practically everyone we met from the UK have been going to Kefalonia for years and most could speak quite a bit of Greek.
Its a wonderful place I just hope the holiday companies dont ruin it.