Pampelonne Beach near St Tropez was spectacular from what I remember. Away from all the glitz and glamour of that part of France however, I also really like Arcachon, pretty town in sout west France, famous for its sand dunes and of course sandy beaches. The Ile de Re near La Rochelle and Benodet in Britanny both fantastic and smart but quite honestly, so many great seaside places in France, spoilt for choice.
In Spain if you want to escape the Costas, San Sebastian is an amazing place with a sandy beach, the weather can be a bit ify, but it's real Spain without the English pubs and full English breakfasts around every corner and famed for its restaurants.
We are going to Cadiz in a couple of months, never been there before, hoping for some culture, authentic Spain, great food and a sandy beach.
Good luck in your quest Splug.