What is the best plan for a day trip from Le Havre to Le Mon St Michels
I would have thought car. I don't know of any public transportation.
Car is definitely the best way. It is about a 2 hour and 45 minute drive from Le Havre to MSM. You could possibly take the train but it will take about 7 hours. I am not sure if there is a direct bus from Le Havre to Pontorson to get to MSM. There is a bus from Le Havre to Caen where you could connect to a bus to Pontorson but that would be a 5 hour journey not including any time spent making the transfer.
Assuming that you're coming off a cruise ship, it's not a day trip unless you have dedicated transportation waiting at the bottom of the gangway.
The drive alone is three and a half hours just to the parking lot and then you have to shuttle out to the mont or hike. Figure the trip up to the Le Havre car place and getting organized is almost another hour.
Back and forth, it's close to ten hours of pure logistics.
For accuracy: Le Mont Saint-Michel