A delightful place to visit two nights is enough as in high season it becomes a little to cosy for me.
It is quite easy to visit from Kefolonia, catch a ferry to Patras and then a bus to Delphi the journey is quite interesting, when returning it is advisable to catch a taxi back to Patras then you are sure to meet the ferry times.
Ask at your hotel the night before to arrange the taxi back.