Notre Dame Inside
It's quite a place inside, too.  The stained glass is beautiful, with the famous rose windows in the north and south transepts.  I always enjoy the feeling in these large cathedrals- considering how long ago they were built.  Of course they do take upkeep to keep them standing- collapses were not uncommon in days of old.  One note here- be careful, as thieves also tour the place looking for your tithes and unsuspecting offerings.