They might still have a beach in the city centre in summer, which has novelty value if nothing else.
Pompidou centre looks fun from the outside. There's also the science museum, the Cite des Sciences
Sciences - Découvrez la science à la Cité des*Sciences et de l'Industrie*à Paris, and the Pere Lachaise cemetery is also great to wander round, lots of famous people buried there and some very elaborate & extravagant tombs.
On the same morbid and therefore highly appealing to small children themes, you can also visit the sewers and the catacombs underneath Paris.
Oh and ignore Disneyland Paris, head for Parc Asterix instead, cheaper, more fun, fewer people and not Disney :-)
And finally given you have a car, it's worth venturing to nearby places like Rambouillet, which has a forest with lots of wildlife, and Fontainebleau, which has some of the best bouldering in Europe, we always used to love scrambling round there!
Err... maybe now someone should suggest things to do in other countries on your itinerary?!
Oh and be careful with scenic routes - they're great, but make sure they're not between tourist attractions. We spent a very very long time driving back to our gite from northern spain via a steep, winding scenic route used by all the tour coaches...
Laura