The towers and ramparts of Aigues Mortes - well worth a visit for the view over salt pans in one direction and pretty canals in the other. In summer this place sweats. Find Restaurant le Minos on Place Saint Louis and order their rouille - it's superb.
Carnac is a megalithic site filled with row upon row of mammoth upright stones or menhirs, just like the one Obelix carries around in the Asterix books. What they were for is anyone's guess. Fascinating to visit, although not easy to get up close and personal with the rocks unless you take an organised tour as most are now fenced off.
The Abbey of Mont Saint Michel. In spite of omnipresent tourist hordes and a climb to the Abbey that will have you huffing and puffing, the visit is well worth the trouble. Just be careful not to leave the car in a tidal car park! (Image from parisvision.com)