We'll be taking our fifth road trip in France, arriving in a couple weeks - late August -and traveling for three-plus weeks in September - staying in Honfleur, Bayeux, Cancale, Concarneau, and Vannes. We've been making road trips in the fall in France over the last nine years (some years we've traveled in Spain and Portugal), but this will be the first time in Normandy and Brittany, and a bit earlier than we usually travel. Over the years we've had mostly dry weather with warm to very hot temps, but I'm thinking we might need to be prepared for cooler weather this time, and maybe some rain?

Suggestions for packing would be most welcome - I want to try to stay on the minimalist side, but I don't want to be cold or wet. After our road trip, we'll be spending a final week in Paris. We will have washer/dryers in almost all our accommodations.

Any other suggestions for "not to be missed" places, restaurants, museums, hikes, etc most welcome, too!

I've holidayed a lot in these regions and can barely recognise the place now, it used to be that at the coast you would expect regular level 8 winds even storms from early September and while that may still continue the warmth now remains longer into the autumn (fall) and the energy in the wind can be pretty strong (due to the subject I'm not allowed to mention).

So generally you should still expect some sunny days and strong winds/powerful rain. But a very wonderful place to visit.

Places to visit, look out for Dolmen all over and the castle at Fougeres

We were there in early October about 10 years ago and had beautiful weather, blue skies and puffy clouds other than the one day at Mont St Michel which had steady driving rain the entire day and at night it was misty and foggy, sort of spooky. I don't recall it being especially windy but weather in general is more windy than it used to be

I like the very short boat trip out to the Cairn de Gavrinis for seeing evidence of prehistoric man. You need to reserve in advance for the ride as well as the site.

September is still summer but everybody knows that the weather is always terrible in Brittany and Normandy.

Yep terrible weather, always, which is why we usually come back with a sun tan, not rust, from our trips.
September can be delightful in Brittany, but there is always the chance, not matter what time of year, of it being wet, very wet, and windy.
Pack waterproofs, but not umbrellas.

My daughter has a T shirt with a weather map of France with rain everywhere but sunny in Normandy. Caption: I have a dream

Great T-shirt!

Is it not odd that the weather services cannot get it correct for three days in advance and still we have weather predictions popping up.
I suggest dressing in layers. If it that weather is totally unseasonal you can always shop for something suitable.