No-one has mentioned Arles yet, so I'll chuck that into the ring. Worth a visit just for the Roman amphitheatre, as seen in Ronin. Then you could drive through the camargue, with its white horses, black bulls and flamingoes, to Saintes Maries De La Mer. From there, head back to Nimes and take the D999 to Sauclieres, and then the D991 through the Gorges de Dourbie to Millau.
Funnily enough, we're doing that in June!
