Full day in Normandy

Impossible. The train must take a couple of hours each way. You've got to get from the hotel to G. St Lazare somehow. You've got at least seven hours of driving if you know the roads cold (and, since you asked the question, you probably don't). Toss in another two hours to pick up and drop off the car, find parking spaces, walk a distance to get up to the attraction, etc. That's close to thirteen hours of logistics before you start to add the time to actually see things.

Thanks Ed! We decided to skip Mont st Michel and Dinan. Do you think Honfleur and Rouen would be possible?

Nope. Almost as much driving. Or if you use trains, it's probably close to a couple of hours to Rouen from Caen and trains don't go to Honfleur. Quite honestly, you're going to be hard-pressed to see just your d-day stuff on a day trip.

If you're willing to be a little less ambitious....a couple of years ago, I started out from Bayeux rather early, took in Omaha Beach and the Cemetary and Museum (this was time extremely well spent, very moving) then took the local roads eastwards stopping here and there (like a site where there are still the remains of German artillery, unfortunately I don't recall the name), spent an hour or two in Honfleur and continued on 'til we got to Duclair, where we were unsuccessful in locating where Camp Twenty Grand, where my father was stationed, used to be. There's a small but very interesting war museum just north of Duclair (open by appointment only, I think).

The only way I would day the D-Day beaches on a day trip from Paris would be with a tour group...and I hate tour groups. This itinerary is absolutely insane. If you want to see Normandy then devote some time to it. Otherwise you've just wasted a lot of time, energy and rental car money trying to do it your self on a quick daytrip from Paris.