Spent close on two years going around europe in a motorhome,its a great way to travel!
I visited Greece on a walking holiday early in the year,mid april which showed the best flora in europe off at its best although higher areas were still snowbound,crossing the metsovan pass the snow was higher than the coach i was on.
The pindos mountains in Northern Greece (inland from igoumenista) holds some of the best walking in europe,of the many highlights
1) The awesome Vikos gorge north of Joanina
2)The Aoos river canyon up near the Albanian border.
3)The magical monastries at meteore.
4) The perama caves circuit.
Theres also many other areas such as the Olympus area and most of the Peleponnese ,for instance theres a great walk around the base of the Mani penninsula in the peleponnese.
Samurai gorge in Crete is easier,more touristy and less spectaculur than the Vikos,if you do it after the vikos you will find it disapointing.Also be aware that the Vikos area has the countries highest concentration of Bears.You will probably see trees marked by them if you keep an eye out.
So all in all a really beautiful country with the wildest mountains in the far north.