The Baptistery of St John in Poitiers is claimed to be the oldest existing Christian building in France. Built in about 360 but much altered, it is worth half an hour (at least) of anybody's time.
For a plan of the building, there is a good link here, and for more information check out Wikipedia
Picture Perfect - The most popular chateau in the Touraine Loire Valley, especially for first time visitors, is Chenonceau. On summer days when it looks like this (below) wh...