definately stop off in Collioure for a visit, the castle is very pretty when lit up at night and theres loads of nice restaurants to eat in. the ones in the sqare or along by the river are the nicest, although I've been told there are nice ones along the beach front too. gets very busy in summer, but september should be ok.
if you're into shopping, there's a place called Le Pertus (spelling may be wrong!) on the french/spanish border. its basically one street, you can get great value on cosmetics, electrical equipment (although I always steer clear of these because you have no comeback if anything goes wrong), designer sunglasses, handbags, sportswear, etc, etc. a shoppers haven.