Get some culture
...do the tourist thing
The best rest day activity is a trip into Paris with it's plethora of museums, galleries, shopping and restaurants, not forgetting the iconic Eiffel Tower. Even if you have a car you're best bet is to hop on a train which will take you to Gare de Lyon right in the heart of Paris.
If you have children with you, or just big kids, Disneyland Paris is only an hour north.
A more local rest day option is to explore the spectacular Château de Fontainebleau. This 1500 room château set in 130 acress of parkland and gardens was home to French monarchs for seven centuries fom Louis VII to Napoleaon III. Well worth a visit.
...Must. Keep. Climbing.
If your rest day is weather enforced and you want to keep climbing there's a new indoor wall that's opened up in Fontainebleau called Karma. It offers both bouldering and sport climbing as well as a pretty awesome looking training area.