Notre Dame dominates one end of the island. It is probably the most famous cathedral in the world and is especially famous because of the book The Hunchback of Notre Dame, by Victor Hugo. The Paris Museum Pass allows you to go up the towers with no extra charge, though you can't skip the line at this site. At the top of the towers, you have a wonderful view and you can also see the famous gargoyles up close.
St. Chapelle is another Gothic cathedral that was originally built to house the "authentic" Crown of Thorns. While Notre Dame took over 200 years to build, this cathedral was built in an unbelievable 5 years. This building houses the most magnificent display of stained glass you will find in the world. It will leave you awestruck.