✔ 最佳答案
Of course you can leave Paris in the afternoon to visit Mont St. Michel, there is 2 buses departing Rennes in the afternoon.
But if you start your journey in the afternoon, you must stay in Rennes or Mont St. Michel that day. The buses departing Rennes at 17:30 and 18:30. The last bus departing Mont St. Michel leave at 17:15. So you must need to find a hotel in Mont St. Michel or Rennes
For more information of the bus service, you can visit
http://www.lescourriersbretons.com/english/lines/lignehoraire.htm#
So, it is much better to leave Paris in the morning and back on the same day
參考: living in Paris for a year before + travel experience