Just a couple of blocks away, and on the same island as Notre Dame, is the incredible Chapel called Sainte Chapelle. The chapel was built by King Louis lX to house relics of christianity. The stained glass windows are breathtaking and are the largest collection of 13th century stained glass in one location. The tremendous amount of glass has been removed at least twice in the chapel's history to protect them from neglect and from the ravages of German invasion. Thankfully they are in their places and the chapel is open for visits. The windows are truly beyond compare.
In the photo above you can see Sacre Coeur Basilica on top of Montmartre. Sacre Coeur stands above Paris on the highest point in the city. The domes of the basilica and the frosty white travertine walls make it shine like a white light above the city. Inside you will find a giant mosaic called Christ in Majesty; it is one of largest mosaics in the world. From the front steps and the courtyard of the basilica, there is a commanding view of Paris. Just as Sacre Coeur is on stage for all of Paris to see, the view of Paris from Sacre Coeur is awe inspiring.
All over Paris there are lovely chapels and majestic churches. Some, such as Notre Dame and Sacre Coeur, are open for tours and are museums, however they are also active churches where services are held on Sundays and holy days. When visiting the churches, you will be asked to be as quiet and respectful as possible, and to remove your hat. You will find people praying or lighting candles for loved-ones, or you might even see a priest holding a service as tourists walk along the perimeter and absorb the atmosphere. All of this just adds to the sense of history and majesty. Many churches, however are not tourist attractions, but they are open to the public and are well worth a visit to experience the architecture, their collections of art and reliquary, and maybe even to say a prayer or attend a service.