Amongst many beautiful churches in Paris, Notre Dame & Sacre Coeur would be arguably the most famous.
Notre Dame, inside, is not consistent to the grandeur outside, but still an easy and free visit, with an ability to also climb to the top (from outside, not from within the Cathedral). The queue to visit stretched across the front courtyard, but moved surprisingly quickly. Entry was free, but 5 euro to visit the Treasury inside. A small price/donation to enjoy the marvelous history in full
Sacre Coeur commands a spectacular position atop Montmartre. On a clear day, the views are worth the visit alone. If you take a train to visit, the train ticket can also be used to ride the Funicular to the top, from which you can then enjoy the walk back down, visiting the Artists Square.
A word or warning though, the touts here are aggressive, and the strip from where Moulin Rouge stands, is essentially a red light district. So long as you know in advance, you can watch your personal security, and choose to visit certain area’s, or not.
