Travel between Versailles & Caen

Taxi from Charles de Gaulle airport to Versailles. Ouch, they are the opposite side of Paris, That will cost mega €€€€€.
Unless you have so much luggage you need porters to carry it, just get the RER (express métro) from Charles de Gaulle airport to Versailles.
Change at St Michel, see plan here: http://www.ratp.fr/informer/pdf/orienter/f_plan.php?loc=reseaux&nompdf=rer&fm=gif
There is more than one station at Versailles, you need to find out which is the correct one for wherever you want to get to,

To get from Versailles to Bayeux, yes you will have to double back to Paris (RER and then métro), but Versailles is just on the edge of greater Paris, so it is not much of a detour.
You need to look up times (on www.bahn.de ) and prices (on http://voyages-sncf.com or https://www.captaintrain.com ).

Mont St. Michel is a bit difficult to get to by public transport, a car may be a lot easier.

Thanks for info. I figured a taxi was going to be expensive....hubby's idea....lol! Will check the train schedules. In Versailles we are staying @ Waldorf Trianon (not sure of spelling).

To plan your movements in and around Paris: http://www.ratp.fr/en/ratp/c_21879/visiting-paris/
If you have a car you could drive around what is left of the Normandy invasion in a couple of hours. If you really want to understand the experience and the history, take a full-day guided tour. That's two nights right there. But you could train it to Caen, wander that interesting city for a couple of hours, see the fortress castle of William the Conqueror, then devote the next day to the WW2 sites. Mont St-Michel is a day on a tour from Caen.

Versailles has 3 train stations, all coming from different locations in Paris. Versailles Rive Gauche is on the RER C which connects to the RER B from Charles DeGaulle at St Michel-Notre Dame station in Paris. Since you are likely to take a cab for the last mile, that is the easiest.
Although there are ways to get from Versailles to Caen with out going all the way back to Paris, they take much longer. From the Waldorf Trianon, Versailles Rive Droite is the closest station and has frequent trains (every 7 to 10 minutes) to Gare St Lazare in Paris where you change to an IC train to Caen. That should take 3 hours.

France trains work on a spoke/hub system and you are going to have to go to Paris to get anywhere else in France especially if you want efficient direct routes. Versailles is an easy train ride from Paris; a taxi will eat your lunch.

Ok, you have convinced us to use train upon arrival to reach Versailles. Would it make any sense to lease car out there & drive to Normandy sites from Versailles? Since we may need car out there anyway.

Well you say you are coming straight from CDG, visit Versailles and then going to Bayeux! Driving is not recommended for safety reasons. I think this will be too much considering you have to deal with jetlag, presuming your flight was a long distance one. Maybe it´s a lot of hussle going back to Paris for the train to Bayeux / Normandy, but once on the way you can relax and you certainly will need that. Versailles means a lot of walking, maybe staying in line for a while too, you need energy for that maybe you won´t have to enjoy the place. So to my opinion it´s better to go first to Normandy with the train and visit Versailles on the way back to Paris if this is the plan.

My wife and I stayed at the Waldorf Astoria In Versailles a few years ago , beautiful hotel , about a 15 minute walk from the train station . The palace is in between the station and the hotel

there is an Avis @ Versailles train station. However, if you have never driven in/around Paris, I really suggest just training back to Paris and then catching the train to Caen from St. Lazare station and then getting a car in Caen, where it should be much less congested. Or joining a tour either from Caen or Bayeaux.

Thank you for your ideas. We are staying 2 nites @ Versailles & since we have been there before will only visit the gardens & fountains. i think we should be up for driving by the 2nd morning. Looking at a map, I thought we could avoid going back into Paris & would mostly travel on highways out to Normandy beaches. We plan to stay 2 nites out there & use vehicle to get to Mont St Michel. We wanted to get the car so we don't have to schlep bags all over.