Well, I disagree with the first post. There is nothing wrong with Paris in August. I spent the whole month of August there 2 years ago and it was very nice. Some days were hot (think 90's) some days were cool (think barely 70's), very little rain and as far as I was concerned nothing important was closed - maybe some small shops/restaurants but very few. August actually has smaller crowds than July or Sept so it can be a good time to go. I think that whole idea of 'don't go to Paris in August' was started many many years ago when it probably was a bad time to go, supposedly all the Parisians packed up and left town for other climes, the idea doesn't fly anymore and I wish people would quit saying it.
As for Provence, I was there in mid-July and a lot of the lavender had already been harvested but I know there are different types of lavender and they do harvest until early August. Also, the sunflowers are pretty much done by then. If that's your main reason for choosing Provence, then it's probably not the best idea. As Janet said it's very hot and dry there that time of year so consider that also.
The riviera on the other hand could be nice then, water would be at it's warmest. I loved Nice and the area around there so that's certainly an option.
You might want to look at the slightly cooler areas of France - Alsace, Normandy, Rhone-Alpes for August. Oh, and I loved the Brittany coast too.