I am planning on spending some time in Normandy and want to visit the D Day Beaches and Mont St. Michel. Is it easier to stay in Bayeux or Caen (or another town for that matter) without a rental car - and does anyone have any tips about the best way to see each of the sites?
Without a car, your best option is probably a local tour bus company. I'm sure they exist, but I know of none specifically.
Battlebus and Overlordtour are two highly-recommended tours that are based in Bayeux. If you book one of their tours, stay in Bayeux.
Train service between Caen/Bayeux and Mont-St-Michel is quite limited. You can leave Bayeux at 07:46 or 17:12 and arrive at Pontorson-Mont-St-Michel at 09:30 or 18:56. Take a bus or taxi from Pontorson to Mont-St-Michel.
Stay in Bayeux, it is smaller and great to walk around. We booked a tour with the Normandy tours, office in the Hotel Gare. We stayed at the Hotel Reine Mathilde. Clean, and inexpensive in the middle of town.
I would recommend staying in Bayeux - the D-day bus tours leave from there. I took Battlebus which was really good - book as soon as you know your dates though. 8 people per van tour.
Also the Hotel Churchill offers transportation for a cost to Mont St Michel - not a bad price either and I think a little cheaper if you stay there.
Easy train ride from Paris