I'm biased, having lived there, but you could consider a trip to Amiens as an alternative. It's an hour from Paris-Nord, and the cathedral is (IMHO) equally spectacular, being one of the many considered to be (by different measures) the biggest in the country. It's best to ignore the station square, but you also get the old Quartier St Leu, the Hortillonage (floating market gardens) and some odd Jules Verne stuff thrown in, as well as some decent regional food.

Then, a detour on the way back can take you via Beauvais, home to the incomplete, but vertiginously tall (IIRC the tallest in France) cathedral, home to a quite wonderful astronomical clock.