When you get there, spend a few Francs on the yellow guide book called The Megaliths of Brittany by Jacques Briard and you won't fall-off matey.
Southern Brittany is "in-your face" megalithomania. Take a decent torch.
My favourite is the flat stone dolmen at Locmariaquer. If you go there, take your cossie and a surf board for some post megalith chillage.
love and life
Fitz