If the forecast is looking bleak, why not spend your day at Archeoscope? You'll be surprised just how much you can learn about the past here.
You'll have sightseeing choices galore in Le Mont-Saint-Michel. Archeoscope is a great place to start here, but there are loads of other worthwhile spots to visit. Mont-Saint-Michel Abbey, Mont-Saint-Michel, and Museum of the Bay of Mont Saint-Michel are near the collection of historical artifacts and will fill in your day nicely.