And if you are in Paris and can't visit Notre-Dame because it is under renovation, maybe go see some other gothic cathedrals that are less than an hour away by train, such as Beauvais' or Chartres'.
For what my opinion is worth, I was awestruck by Beauvais' cathedral personally (even though, or especially because, I visited a fair number of period churches). Its nave is incredibly high, high enough that the aisles vaults by themselves are higher than some entire churches. It's also only halfway done, which is somewhat disappointing from an architectural perspective but fascinating from an archeological perspective: half the Carolingian cathedral is still there, which is a rarity.
Another beautiful cathedral around Paris is Amiens.
It's a more recent cathedral than Notre-Dame, and consequently higher, nearly as high as Beauvais in fact, but this one didn't collapse (even if she needed a very costly iron belt to stay that way).
It's also located near the quite nice Saint Leu neighborhood with tons of restaurants near the main canal. And right next to Saint Leu, you have the Hortillonnages (crisscross of waterways and small gardens) which are also worth a visit (it's possible to rent boats to visit them).
It's only ~1 hour away from Paris (Gare du Nord) by train, and definitely worth spending a day there in my opinion. Just don't be afraid by the sight of the tour Perret when exiting the station at Amiens ^^.
For what my opinion is worth, I was awestruck by Beauvais' cathedral personally (even though, or especially because, I visited a fair number of period churches). Its nave is incredibly high, high enough that the aisles vaults by themselves are higher than some entire churches. It's also only halfway done, which is somewhat disappointing from an architectural perspective but fascinating from an archeological perspective: half the Carolingian cathedral is still there, which is a rarity.