Please advise us as to the most convenient and economical public transportation directly from the CDG airport to Mont St. Michel. Is it better to fly to Rennes or take the train and/or bus? We will be arriving on a Sunday morning in May. Thank you.
If there is a convenient flight to Rennes from CDG on a Sunday morning then I'd fly. Otherwise, you will have to get to Gare Montparnasse in Paris and take a train to Rennes and take the bus from there. There is a bus that coordinates with the train but I am not sure that it runs on Sundays. You may end up having to rent a car for the trip from Rennes to MSM.
I do not know when you are going but Air France lists 3 flights Cdg to Rennes on Sunday May 7 2017
9:25 am
1 pm
and
8:45 pm
and those are currently going for $53 one way which is less and faster than taking the train from the airport all the way Rennes;
then you can take a regional bus directly to Mt St Michel from Rennes.
If you book your transatlantic flight with Air France or Delta, the short flight to Rennes from CDG would be on the same ticket.