We had a fab day at the beach - well a beach by a lake. About 1 hour from Roussillon is the Etang de la Bonde lake. A stunning setting and one can park the car alongside the lake by going into the camp site next to it (€2 parking I think it was). Make sure that it is the camp site right next to the lake as there is another just nearby which is not the right one... From memory the one we wnet to is Camping de la Bonde.
The swimming is lovely and there is a cafe serving not bad food (though self-service). We took a picnic and then got some beers / wine at the cafe.
To get there drive to Bonnieux and then onto Lourmarin towards Ansouis.. this is the full address.