Thats a hard one... There's nothing relevant for sale on the net, I didn't even find anyone mentioning having seen something similar (that works, that is) and is sorta plug'n'play.
The simplest ( cheapskate ) way to do it is to get a seperate sound recording device, and honk or sth after you turn on both the camera and the mic, so as to be able to later on mix them together in your pc using the honk as a synchronisation aid.
Better yet, if you are a little aquainted with electronics basics, you can hack the camera open and splice a mic in the built-in mic line. I've seen a case where this method worked just fine, and one where it didn't - it actually failed big time. Though the second one was a 20$ shitty cam from fleabay.
Nothing else comes to mind atm.
What does, however, come to mind is lots of advice about the last part of your trip, Greece; scenic back roads, places worth visiting, places that arent on the map etc.
I dont visit the forums often , so if you yould like any info mail me at asterios25(at)hotmail.com