The Basilique Saint-Michel is a Gothic gem built between the 14th and 16th centuries. Its charms include an altarpiece depicting the triumph of Michael, its patron saint, over the dragon and some fine stained glass (although a lot of its stained glass was destroyed in WWII). It's also a stop on the Santiago de Compostello pilgrimage.
But the church itself is overshadowed by its freestanding belfry, which at 114 meters (374 feet) somewhat grabs the attention. From June to September, it's possible to go inside the tower and climb to the top. It's worth it for the views over the river.