I love September in Paris, except it's sometimes hard to get a room, especially if you're there during fashion week.

Springtime (May, June) is great, especially if you go to Giverny (and you could do a bicycle tour there as well!). All the flowers are in bloom. Same for Provence. September is too late, all the lavender is gone and the sunflowers are dead.

I don't mind crowds, and I'm happy in the south of France along the shore in summer. But September on the Cote d'Azur is fabulous, since you practically have the beaches to yourself and the sea is still warm.