The GR 20 on Corsica , the GR 5 from Geneva to Nice and the Pyrenees Haute route are all fantastic mountain treks.
In Spain the Picos de Europa offers a great mix of high mountain and low level trekking.
For a flattish low long distance route – The Romantic Road in Germany from Wurzburg to Fussen (Austrian Border) is good.
For a more chilled trip go Island hopping in Croatia.