What a great trip that will be! First, what is she interested in, and what do you consider "the obvious"?
The guignols at the Jardin de Luxembourg? Saling the little boats in the parks? Parc de la Villette? The Bois de Vincennes with its zoo, boat rides, and nearby château? Hot chocolate at La Charlotte de l'Isle? Museum-wise she might enjoy the Cluny Museum--it has a wide variety of displays and isn't so big as to be overwhelming (and if she's read Harry Potter, she may notice Nicholas Flamel's tombstone there). There's the Café Zoïde which promotes itself as the first cultural children's café (0-16 years, parents of course being welcome too):
http://www.cafezoide.asso.fr . Finally, here's a French site that lists lots of things for children:
http://www.occuper-ses-enfants.com/. The Ferme Pédagogique de Paris looks interesting... along with seeing sites, would she enjoy interacting with other children?