On Sundays at 4:30 PM the Notre-Dame has free auditions d'orgue, which I think means organ auditions but could just be French for organ concert. Either way, if you're in Paris on a Sunday afternoon, swing by the Notre-Dame around 4 PM to get a seat. Wait patiently; admire the soaring height of the cathedral's ceiling; look pious as the mass of tourists tramp by, and prepare yourself for the thunderous giant organ filling the old stone walls with the works of Bach, Mendelssohn, Liszt, and others. Donations are welcomed.