This area of France is one of the best organized for biking. Here are some programs that use mainly cycle paths.
 

Depending on your choice, you wil ride through the Marais Poitevin along its little canals to the Ocean. There , the blue ligne of the horizon will only be interrupted to let you discover the charms of Ré island and La Rochelle old town.Then you will ride along the coast to Rochefort, Oléron and Aix islands.
Finally, you will enjoy cycle paths of "La Grande Cote" from La Palmyre to Royan. There, you may take the train back to you starting point.