Cathedrale Notre Dame de Paris: climb its 400-odd spiraling steps for magical rooftop views. http://www.notredamedeparis.fr/en/

How much time? taken from tripadvisor:

• Somewhere between 15m-2.5hrs, I’m guessing 1hr sounds like a reasonable compromise. Link
  
• Towers’ line is pretty long so depends when you arrive as they let ~20 persons at once.
  
• Cathedral line goes fast. Inside visit depends on your interest, about 1.5hr.
  
• The interior is free & easy to visit. Lines can be long but they move very quickly. I typically budget .5hr for the inside of the cathedral.
  
• The towers have very long lines, and they cost money, and they involve an exhausting climb inside a claustrophobic stone stairwell. The wait is variable but it could be 2hrs or more in some cases. The staff is surly and the capacity is limited. Not for people with a fear of heights or health issues. Nice view and interesting sights, though.
  

The cathedral is open every day of the year from 8:00 to 18:45 (19:15 on Saturdays and Sundays).

The entrance is free

Audio guides are available at the reception desk, entering the cathedral. You can thus, through a journey of about 35 minutes, discover the spiritual message of this high place of Christendom, through its history, its architecture, its sculptures, its stained glass windows … 

– Monday to Saturday: 09:30 to 18:00 
– Sunday: 13:00 to 18:00

NEED TOWER DETAILS

Paris Museum Pass? Not sure about this