Hi Vaughn,, while a daytrip could be done from Paris to the D Day beaches. it would be much much better to plan on an overnight.. reason is distance.. if you take the train from Paris you will likely still arrive after daytours of the sites have left,, but you could rent a car and drive yourself to a site or two..
However.. there are big bus companies that do daytrips there.. its a long day( on the bus by 6.30 am.. back by 8 or 9 at night) but you can sleep on the bus and not worry about figuring out how to get to things etc.. they also are pricey.. ( over 150 euros per person at least) but there are two reliable companies that I personally know of ..
Pariscityvision.. run via Grayline.. just google "Pariscityvision" for site. Its a pricey tour.. but its not really any different then the other company that is cheaper..
Francetourisme( yes.. spelled that way).. they run about 115 euros per person...
I have used Pariscityvision for a tour to Loire Valley Chateaus..and I have a friend who used Francetourisme for Normandy D Day beaches and Brugges..
Doing Loire Valley is possible as a daytrip,( although there is so much to see a night or two is worth it there if you can spare it) , if you are will to rent a car.. or you could take the train from Paris to Tours.. and then there pick up a small van tour to chateaus.. taking public transport to see chateaus is not very easy.. buses come infrequently and chateaus are of course spread out. I have been to Loire Valley twice.. once I was alone with my child( so I had no desire to rent a car with a child as navigator!) .. so I took the bus tour( daytrip from Paris).. and another time I was with my father who is French and was happy to drive there.. ( we also stayed 3 nights in area) .. so with a lot if up to you ,, how much time you have.. and what you are willing to spend.. If time is short and money not tight.. then bus tours would allow you to do both as rushed daytrips..