How about the French Pyrenees? Tremendous walking on and around Pic de Canigou (2784m), mountain scenery in the valleys of the Tet and Tech rivers, delightful & historic villages (such as Villefranche le Conflent) with good restaurants and bars, hot spring spa (St Thomas-les-Bains), duty-free shopping in Andorra and artist-friendly coastal ports such as Collioure.... Fly Ryanair to Girona or Perpignan (2hr flight) & hire a car. It can get very hot & crowded in the summer, and it can be wet in the autumn. best time to go is in May or June.
I'd also suggest Bruges - easy to get to via the Eurostar and Belgian rail - wonderfully romantic, excellent restaurants, beautiful architecture, fantastic museums, dreamy chocolate and great Belgian beer!
Time of year is very flexible as, being a city break, there is plenty to see and do even if the weather is inclement. But I would imagine that a little before Christmas would be nice as the Burg and Markt have, I beleive, Christmas Markets.