For La Rochelle and area:-
The Aquarium at Port des Minimes - reached by car or boat
The weather ship museum
Walking around the towers and having sea food at the port side
Visit the Ile de Ré - you will need to pay a toll for the bridge but it's worth it
Fort Boyard offshore - ask the Tourist Office
Fouras Plage/Chataillon Plage for swimming
Saintes - historic site and market
Cognac - for the cognac tasting!!
Marais Poitevin - inland - take boat tour, and get someone to light the methane on top of the water!
Niort - Donjon museum
Really inland (but worth it!) the FUTUROSCOPE science park
And further north, the Zoo at Royan
....................... for starters!