Would picking our rental car up at Paris Gare du Nord when we arrive from London @3PM and driving direct to Mont St. Michel be crazy on Sat. mid Sept.? The earlier eurostar trains are more $ unless we leave London really early. On Sunday, we would go to Bayeux or nearby town (for our 2nd night)to do the D Day tours & museums on our own (no tours available)then sightsee on our way back to Paris on Monday. What time should we plan to return our car and avoid rush hour traffic? We will be staying in the 7th arrond. Thank you.
Assuming you're in the car and moving by four pm, since it's about a three and a half hour drive (pushing it), you'll get to MSN by seven-thirty. Sunset will be around eight; it will get dark within about an hour at that time of year. Saturday will not present a traffic problem.
Coming back into the city traffic won't be a problem either (anytime in the afternoon or evening) since you'll cross the Peripherique (rather than having to drive on it) and essentially scoot right down the Champs to one of the bridges near the Invalides and pop into the 7th Arr.. Keep this pattern in mind when you pick where to drop the car.
The preceeding assumes you'll drop the car near where you're staying. If you return it to Paris Nord, it's going to be a bear anytime after three pm. If that's your scheme, let me know and I'll tell you generally how to do it, but it takes a lot of typing and I have slow fingers. Also, it will bore other readers to tears.