The best thing with Kefalonia was the beaches!!! You find there all kind of beaches: rocky, sandy, remote, busy, with beach bars or nothing and each one is great and uinique. No need to talk about Myrtos, who doesn't know this gorgeous beach? However, there are many others in
Kefalonia than Myrtos. Antisamos, Emblisi, Poros, Kaminia, Xi, Koroni, Megali Ammos...
Do not waste your time with buses. You will lose most of the day packed and waiting under the hot sun. Rent a car and make the tour of Kefalonia. If you book early, car rentals are really cheap. I found in Greeka a nice Hyundai Atos only for 100 euros for 4 days!!! Of course, I booked three months earlier, but so did I with the air tickets and the hotel.
Eat a lot and swim as much as you can. The prices are a bargain here compared to other Ionian islands, like Corfu or Zante. Four people we never paid more than 50-60 euros for a meal. Argostoli, the capital, was nice for an evening walk but too busy. Lassi was too touristy, Lixouri was fairly nice, but the best village for more was Fiscardo. It was certainly the most busy village in Kefalonia but so nice with all these colours, the narrow paths and the port with the yacths. Ohhh, I miss Kefalonia so much!! Certainly will go back this summer or the next.