In the middle of Paris you'll find the beautiful cathedral Église Saint-Eustache. It is located by the entrance to the old markets of Paris (les Halles) and at the beginning of the famous "rue Montorgueuil". It is yet one of Paris' Gothic gems and was built between 1532 and 1632.
The church is an example of Gothic architecture which had renaissance details added. It's an impressive building. It is 105 metres long and insider the ceiling is 33,45 metres above you. Outside the cathedral displays a beautiful mixture of Gothic and renaissance styles, which a front sober and heavy and a backside filled with classical details and decorations.
Within the cathedral is the largest pipe organ in France, greater than those found in Notre Dame and Saint Sulpice. The organ was originally built by Ducroquet and was later changed by Joseph Bonnet. There are 8000 pibes.
The cathedral has been quite famous during the years and both Louis XIV, Mozart, Anne of Austria, Turenne and Mirabeau, Marie de Gournay, Madame de Pompadour and many other have used the cathedral or been burried here.
It is certainly worth a visit, especially due to its architecture, it is very impressive. And while you're in the area, you might as well visit the large underground mall beneath; Forum les Halles.
Metro/RER stop: Chatelet / Les Halles