Wherever you go you better book soon as you're pretty last minute to be booking a beach vacation place only a month from now. Just because I have some trip reports about them you can have a look at two of my trip reports (that have lots of photos) about the areas of Brittany in The Gulf of Morbihan and The Emerald Coast (near Saint-Malo).
There are great beaches all throughout Brittany so you won't go wrong choosing any region in Brittany. Actually, I find all of the Atlantic coastline to have nice beaches and coastal scenery.
There are great beaches from Saint-Malo heading west towards the Côte de Penthièvre. Towns here (among many) to consider could be Saint-Malo, Dinard, Saint-Jacut-de-la-Mer, Saint-Cast-le-Guildo, Sables-d'Or-les-Pins (or anywhere along the Cap Fréhel peninsula), Erquy and Le Val-André.
In the Gulf of Morbihan there are many long stretches of sandy beach so any of the towns near there are good (Carnac, Locmariaquer etc.) Good beaches abound in Brittany.
If Annecy interests you I spent two weeks there last August and I enjoyed swimming in the lake immensely. Here is a report about Annecy and environs: http://tinyurl.com/3awectn
As I said, I would make a choice quickly with such a short time until you arrive.
Edited: 7:22 pm, June 11, 2012