One option is to contact the Rhodes based schools/aeroclubs in case they can meet up with you at nearby Kos for a flight.
Check for LGRP based operators here
http://hellasga.com/links.html
Other option is to contact John Chaskakis who has based aircraft in Kos and some CFI/Examiner licenses, email haskair[at]yahoo.com
I have not communicated with him for ages and I don't know if he still operates from Kos with his Piper Seneca.