Todd,
Visiting Normandy and the D-Day beaches from Paris is not ideal, due to the travel times involved. While it can be done, that only allows enough time for a brief view of a few of the major sights. Some of the day tours from Paris can also be expensive, such as the small group tour offered by Paris City Vision.
You could take an early trip to Bayeux and sign up for a half-day tour with Overlord Tour or one of the others (pre-booking the tour would be highly advisable). After the tour, have dinner in Bayeux and then return to Paris in the evening. It will be a LONG day.
There's also an excellent WW-II Museum in Bayeux, as well as the famous Tapestry that describes a much earlier battle, but not likely you'll have time to see those on a day trip.