My trip plans keep getting torpedoed. Now considering France for 3 weeks mid September into October. I assume Paris will be crowded but reading online it seems that most of the country is considerably less busy than in summer. Have you been to France in September recently?
September is the most expensive season for Paris and it is very popular throughout France as it is lovely weather. It is a bit less crowded than July or late June but it is still quite busy. I am traveling in southern France this fall in September and a couple of places I hoped to rent were booked up long ago. We had to re arrange part of the trip because the place we wanted was booked then and were able to get it a different week. I think it is the nicest month to travel in France.
We travel to France in September and October.....our favorite time also. As Janet said, it is a busy time in Paris, but we love Burgundy and often times can get wonderful deals in gites in small villages. The vineyards are in beautiful color in the fall and driving is great during those months.
We've been to France in September-October and also in April-May. I'd say the spring months are less busy but the fall is less busy than the summer. September is a busy expensive month in Paris but not, I think, so much elsewhere. I think you'll be fine if you plan ahead for lodgings in larger cities or popular destinations. And, as with all travel, be prepared for changeable weather.