France Southern Coast (central/non-Riviera)?

There are a few areas with good, but rarely great, sandy beaches. From East to West:
- south of St Tropez (Pampelonne, Cavalaire). Avoid St Tropez itself at all costs between 15 July and end August, crowds are difficult to handle.
- Hyères/Giens, some long sandy beaches, less fancy than around St Tropez. Hyères itself is inland but an interesting town.
- la Ciotat does not have great beaches (there's a pretty cove and an average, narrow sandy beach), but it is a charming harbor, still relatively undiscovered, less crowded than Cassis and with good access to the Calanques and Cap Canaille
- some good, wild beaches in the Camargue area: just east of Saintes Maries de la Mer, or Espiguette beach right south of Aigues-Mortes (which is a great base). Beware of mosquitoes in Camargue (not a problem on the beach, but inland). Also, remote areas tend to be clothing-optional beaches.
- the resorts right next to Montpellier (la Grande Motte, Palavas...) aren't very pleasing to the eye, and get very crowded, but the beaches are good.

As you can see, it is a mixed bag, and I would treat beach opportunities in that part of France as a bonus rather than a focus.

Despite normally agreeing with 99% of what Balso says, I like Palavas-les-flots but go out further toward Maguelone Cathedral. Petit Travers, Grand Travers, Carnon are all nice and sandy.

Grand Travers is indeed lovely, and I confess that I am not exactly a Languedoc 'expert', so perhaps I should give Palavas a second chance ;).
I forgot to add that Montpellier is very interesting to visit as well.

What I love about the beaches is the restaurants out on the sand in the summer, called paillotes in French. A cool drink, fresh oysters from a nearby oyster farm....

Just to add to balso's (eta and bets) reply

The key with Montpellier beaches is to avoid the ones closest to the public transport links, as they get the most crowded.

Sete is a seaside town with long stretches of soft sand beaches nearby. It is west of Montpellier. Just to note, further away to the west is Cap d'Adge, which is a popular naturalist resort but also has some nice beaches. As called out -- Espiguette is the large, expansive beach nearby and may be one of the best. it retains its somewhat wild character.

Sanary-sur-Mer is popular with the french. Bandol is nearby, and the area is famous for rose wine. A lot of these beaches tend to be very narrow and not nearly the wide expanse of some of the ones called out.

I agree that Hyeres is a charming place to stay -- there are a number of smaller beaches in the towns along the scenic route between Hyeres and Saint Tropez.. Ramatuelle is the petit village near Saint Tropez. The nearby Pampelonne beach is a long stretch of white sand and chic beach clubs. It is famous because it is rather nice, but it is popular. What's I like here is there is no line of concrete buildings backing the beach, despite the services (beach clubs, restaurants, etc)

@Gooster, funny that you mention Sanary: I am heading there this weekend, I go quite often (understatement).
I thought about it while typing my list. It is super charming - and so is nearby Bandol, to a slightly lesser extent - but I understood the brief here as "sandy beach", and while there are a few, they aren't exactly the best. Still a great place to stay! Parking is nightmarish from mid-July to late-August, though.

@balso enjoy your time in Sanary! Yes, I threw it in for the charm of the town, even though the beach is not great. But not all of it is rocky/muddy, but there are definitely better choices.

Thanks everyone!

I'll be checking these out.