Sunday (last) and the museums are free. So, it would be remiss to fail to take advantage of such an offer.
The Musee du Quai de Branly sits on the left bank near the Effiel Tower. It houses the most wondeful collection of ethnographic works from four continents, arranged by geography and theme. The building itself and the lovely garden is worth the trip alone (bar the piped birdsong - why?).
Imagine if you had to redesign the Pitts Rivers Museum in Oxford for the 21st century and this might be what you'd come up with.
The current and very scary Shamanic exhibition is good value too if you're into voodoo altars, skulls, feathers and fetishes of all kinds. Gothic run riot around the world.
My personal favourites were the Australian aboriginal paintings and Amazonian feather head dresses, but there's too much to choose from really.
Details here: http://www.quaibranly.fr/