From Paris... Is Rouen / Giverny / Mont St Michel and back to Paris a reasonable day trip? Also, I am assuming auto is best mode for party of 5... recommendations for reliable auto rental appreciated... THANKS ! jfm5x.

No, we rented a car on our way to Bayeux and our tour of Normandy. We planned to stop in Giverny, but we never made it due to a traffic jam.

Giverny, by itself, is a day trip from Paris.

It depends what you enjoy, but if you are pressed for time, I would skip Rouen. It's a big city.

Check the mileage. It's quite a drive to Mont St. Michel, and it's crowded in summer. We spent one night at a rural, historical, nice B&B, Les Blotteries in Juilley, France.

I like Autoeurope.com for the car rental.
I don't think this is all possible in one day. You could do leave early, tour Giverny, drive to Rouen and then spend the night there. Then head towards Mont St. Michel in the morning. You wouldn't see much of Rouen but you could see the some of it. If it were me, I'd want to spend another night near Mont St. Michel before heading back to Paris. But perhaps it is possible to get there, tour it, and drive back to Paris. I probably wouldn't do it, but that's me. If you arrive at Mont St. Michel during the day, you will be part of the mass of tourists and it is going to take far longer than you expect to even reach it. I agree, check the mileage because I think it is further than you think.

It's almost a 4 hour drive from Paris to Mont St Michel, without stopping. You'll be driving 8 hours .It takes several hours to see Giverny alone. A one day car rental is also very expensive.

I forgot to mention that we rented from Hertz. GPS was very helpful, especially when you miss a turn at the roundabout.
We picked up the car at the Montparnasse rental office in Paris.

To visit all 3 sites, you had better spend one night away from Paris. The best rates on www.autoeurope.com are always from 3 days.

All, thank you for this info... sounds like i need to revisit planning on this one ! your responses very appreciated, best, jfm.

You could take the train to/from Rouen. We enjoyed looking around Rouen very much and felt it was well worth a visit. It would make a nice one day out of Paris visit.