IMO the best place to stay for the DDay beach sites is Bayeux. However, coming from Paris, I would take a train to Rouen, stay at least one night (if you get there by mid-afternoon). See the clock, cathedral, wander in the medieval quarter and enjoy a nice meal. Pick up your car next morning to start your Normandy exploration. We drive on D982, along the Seine river to Bayeux. A stop at the Abbey of Jumieges is very worthwhile and unforgettable (spend about an hour or two there). You will get to Bayeux in the afternoon. You can drive to the sights yourselves or take a tour or two. For me, two full days there is plenty, but some posters are more familiar with the area and cover more territory. They can advise you better. I do suggest exploring the lovely town of Bayeux and visiting the museum and cathedral there. (This could be done on arrival in Bayeux, depending on what time you get there.) Alas, the tapestry is unavailable at this time. Make sure your lodging has some parking available or nearby.
The drive from Bayeux to Entretat could be leisurely, stopping aong the way at any number of towns, churches, abbeys, etc. As to car return, perhaps Le Havre (a bigger town) would have better options for open car rental returns. I would check that out, plus train options for your trip back to Paris, where I assume you are headed. W
hen you decide how many nights you want in each location you can check out the Le Havre option. It is only 30 minutes or so from Entretat.
Have you gone to Auto Europe to compare car rental options? They are a good, reliable place to see what's out there for car rentals.