Guillestre in the Hautes Alpes region (a couple of days easy driving from Calais/Dunkirk) is a lovely place to stay. The camp site run by Monsieur Pascale is just above the main town, is fairly basic (comfortabe though, not primitive) and is really nice and relaxed.
It's over 1000m up, near loads of lovely Alpine walks, St Veran (highest permanently populated place in Europe) and the town itself has good restaurants and a market.
Beware – there are loads of trail heads just up the road, got me chomping at the bit. I'm sure you could hire a bike. You can head to the South coast pretty easily once you've had enough of the alps too.