Ten years ago in June we took the train from Paris to Carcassonne and stayed for a few nights. Then we picked up a hire car and drove back to Auxerre stopping at various places en route, where we dropped the car and took the train back to Paris.
Highlights en route - don't know of any of these are doable by train - the Foster designed Millau viaduct, the castle they are building at Guedelon using 13th century methods, the volcanic scenery Puy de Dome just outside Clermont-Ferrand (the dining experience Cuisine Degustation at Hotel Radio never to be forgotten) and views of Eiffel's Garabit rail viaduct. Would like to ride over that on a train.
We arrived in Auxerre mid afternoon and dropped the car and only had the rest of the day to explore before leaving next morning by train for Paris but have always wished we'd stayed two nights so we could have explored Auxerre more.