One of your first stops should be Office de Tourisme de Saint-Cast-le Guildo. Here, you'll be able to orient yourself and get some great tips before you start exploring.
When you've finished at this tourist information center in Saint-Cast-le-Guildo, why not make your way to Cap Fréhel, Fort La Latte, and Plage de Pen Guen? They're close to Office de Tourisme de Saint-Cast-le Guildo and also a hit with tourists.