I lived in Paris for almost three years and could give suggestions for the city itself, but unless I’m misunderstanding the OP the question is about things to do **near **Paris and not in Paris itself. Please correct me if I’m wrong
Lourdes is on the other side of the country, but still a doable trip if you’re willing to make the effort. It’s worth it. Flying is fastest, about an hour and 15 minutes from the Orly airport (south of Paris) to Tarbes (15 miles from Lourdes) or Pau (36 miles from Lourdes but the flight will probably be cheaper). There is also the high-speed train (TGV) from Gare Montparnasse in Paris. The trip takes about six hours but is much less of a hassle and will drop you directly in Lourdes.
If you choose to do this, however, allow at least one full day in Lourdes. I was there at the end of April/beginning of May for three, and it wasn’t long enough in my opinion. My pastor won’t go if he can’t be there for at least a week. YMMV.
Closer to Paris, there is the aforementioned Lisieux, Nevers, Vézelay Abbey, and any number of cathedrals: Chartres, Reims, Laon, Amiens (tallest complete cathedral in France, surpassed only by the incomplete one in Beauvais), Rouen…
Mont St-Michel is a good choice, as is Saint-Malo (fantastic walled city in Brittany; the cathedral is lovely but I wouldn’t go there just for that). Be aware that both locations will be absolutely overrun with tourists in July and August. Saint-Malo has a summer population of 200,000+; the rest of the year it’s 45,000.
I’m in a hurry right now (Mass in an hour, still haven’t showered

) but if I think of anything else I’ll let you know.