Hiding in plain sight near Les Halles, just down the street from the more famous and annoyingly crowded Notre Dame, you'll find what I think is Paris's most impressive gem: L’église Saint-Eustache.
She's almost 400 years old, has vaulted interiors over 100ft tall, and was used as a barn during the French Revolution. Her giant windows let in swaths of light while the ginormous chandeliers provide the rest. With a mix of gothic and renaissance styling she's not particularly pretty from the outside. However, once you step inside you're transported to another world. But what really makes my jaw drop is the organ - with 8000 pipes it's the largest in France.
And it's always empty.