So am I on Tuesday for 2 weeks from Cardiff. Been there before...its stunning. If you are young and want clubs etc go to Lassi and Argostoli as they're the liveliest places. The rest of the island is unspoilt and stunning. Been to Greece/Greek Islands probably 20 times??? Kefalonia is the best of all. Tourism hasnt killed it yet.
Go to the Dringaki Caves....an underground lake whose name escapes me....thats a great experience, Fiskardo in the north....a beautiful fishing village and go the Myrtos beach where Captain Corellis mandolin was filmed.....incredible.
Your best bet is to hire a car and search for yourself.....its relatively small so you can see it all in a couploe of days.
Its greener than most of the islands because its further north.........it shouldnt rain this time of year....should be about 80 degrees.