I'd go to the exhibit on Bohemes at the Grand Palais. This exhibit is dedicated to the myth of Bohemia as the "home" of artists, starting in the mid-19th century. The exhibition includes the work of such artists as de la Tour, Degas, Corot, Turner, Matisse, Van Gogh and Picasso and traces the intersections of artistic life not only in the visual arts but in literature, criticism and music.
This is one of the new brand of exhibitions curated to connect grand themes across and through and between artists and media and styles.
There's a music festival and exhibition dedicated to Django Reinhardt starting tomorrow, as well. At the Cite de la Musique, it includes concerts through next week. Reinhardt was the jazz guitarist who introduced and made world-famous that distinctive French sound evoking French gypsy music. Reinhardt was born into a family of gypsies living in Paris, and started recording as early as age 18.
By the way if you are a fan of the film Chocolat, you'll recognize the style.
And yes, in both cases, I'd wind up the day in the bookstores.
But this is the perfect day to be walking along the Seine or through the Jardins du Luxembourg, to enjoy autumn. To take a lunch of French baguette, cheese, grapes or figs and an Orangina or split of wine, along with your best book and a camera. Yum!