I agree with Dean -
Fiskardo is the where the oldest architecture is found on the island as it was the only town that was left relatively undamaged after the 1953 earthquake.
Assos is spectacular and great for a very peaceful rest, but it can be rather isolated if you don't have a car.
Sami is also lovely and there are quite a few tavernas and bars now since the movie came out.
On the south coast, you would probably enjoy Katelios very much - it is probably the resort that more people come back to than any other on the island. There are a handful of great fish tavernas and a few bars. Apart from that, it is peace and quiet.
But for me, the best resort of all is Poros, with its long stretches of beach. It has a harbour and a marina and a village square and loads of good, cheap tavernas. There are bars to suit any taste and the locals still treat you like a long-lost friend.
But, wherever you choose, I am sure you will really enjoy Kefalonia.