I had such a holiday when I was a kid, though much of my memory of it is sketchy. Clambered over all kinds of concrete emplacements, towers and wotnot. Also remember an underground hospital or something like that.
My abiding memory of the trip though is watching the Iranian Embassy siege in a TV shop window. For months after that we were always the SAS when playing army!
Great place for a trip, though I must admit that I prefer 'proper' France to the little bit of it that is still in the hands of the British Crown.