The only thing to really do if you need to do it all in one day is to take the train to Caen. Make a reservation with the Caen Memorial Museum, they will pick you up at the station, and then take you on a 5 hour minivan tour of the Normandy beaches, with a tour guide in English. They give you lunch, and then return you to the train station. You can also tour the museum, a wonderful place with loads of exhibits and artifacts from the D Day invasion.
See "The D Day Tour" here:
http://www.memorial-caen.fr/portailg...er&Itemid=1745