Ile de Ré
St. Martin de Ré is probably the prettiest fort/port in France, and is the place to be based for explorations.
The island is best discovered at a slow pace on a cycle. This allows for an unhurried appreciation of the villages, numerous market gardens and, of course, the sea.
Returning in the evening you will find a hive of life in St. Martin, a perfect place to enjoy a glass of wine and watch the people come and go.
Like for Noirmoutier avoid August.