I wouldn't, frankly, bother with Carcassonne. The 'medieval' city looks intriguing from the autoroute, but is disappointing in reality. Saved from demolition in the Victorian era, and extensively reconstructed, it has about as much soul as Disneyland.
If you were thinking of heading in the direction of Montpellier, I would recommend a couple of days in Avignon. As ancient walled cities go, this is the real deal and it has something for everyone: bars, restaurants, shopping, live music, the Palais des Papes, Pont St. Benoit (altogether now, "Sur le pont d'Avignon ...").
I first visited Avignon in May 07, alone, when picking up my 1200GS from having its fuel pump controller replaced. In October, I went back with my OH and some American friends. Everyone loved it.
Not far away you also have picturesque hilltop villages like Roussillion, the Vaucluse wine region and, for warmer months, lavendar fields and olive groves.