Notre Dame Cathedral - On the first Friday of every month at 3:00 in the afternoon at Notre Dame the Holy Crown of Thorns is displayed. There is a short service and then all who wish can approach the relic to view (or kiss, if you wish to). I attended last June; the service and procession took about two hours. It was unique and very moving.
Notre-Dame Cathedral. Aside from the rose windows, I've never seen the attraction here. What I AM a fan of is the view. Get in line an hour before the towers open - grab a pastry from the shops surrounding and chill. There is no way to skip the line here - so being at the front will guarantee you immediate access - otherwise you'll waste lots of time here - and the view is worth it!