You absolutely have to visit the Musée National d'Art Moderne, which houses the largest European collection of modern art with works that span the entire 20th century. Your tour begins on the fourth floor, and keep in mind that although the exhibition is called "permanent", the works are rotated and often change, considering how the museum has 30,000 paintings and sculptures available.
Remember that your admission to the modern art museum also gives you access to the panoramic terrace. Even though it is only six floors tall, the Beaubourg is surrounded by shorter buildings that significantly open up the panorama. So take the outdoor escalators to enjoy a view of Paris from above!