Rainy Weather Ideas

If it's raining, indoor activities I suggest :
Galleries Lafayette department store, fascinating interior, and they have a rooftop deck, cafe, etc.
Angelina tea room, on rue du Rivoli for hot chocolate.

Stroll the Marais if not too rainy, many fascinating shops and cafes
I would hope you get to at least one museum, they are the best indoor sightseeing in Paris -- and hopefully she might like one!
the D'Orsay and Orangerie are my recommendations, (perhaps the Picasso?)
Go to top of Montparnasse tower, if weather is fairly clear, the indoor observation deck that has great view of Paris.
Of course Parisians live in the rain and go everywhere, just bring an umbrella with you at all times and go for it!

Take an umbrella, good shoes, and head out regardless - it isn't likely to rain every moment of your visit. Paris in the springtime is beautiful regardless of the weather! Get a museum pass so you can dash in and out (and use bathrooms). Even if she doesn't think she likes museums, try checking out more accessible ones (the Musee d'Orsay is one of my favorites). See if you can work horse theme into any art museum (see this example by Degas). Ride the metro. Pick some off-the-beaten path sites that might spark her interest. I personally liked the sewer museum for its weirdness...and it's totally under cover, so to speak. Take the bus 69 tour (see RS book) for a tour of the city without getting wet. Hop off at the Père Lachaise Cemetery, really beautiful in the rain. Find a cozy cafe and eat croissant.

There's riding in the Bois de Boulogne and Bois de Vincennes, on opposite ends of the city. But maybe not so much in the rain. Besides all the art, the Louvre has the apartments of Napoleon III (Richelieu wing on the north side) with over-the-top red velvet everything everywhere. Likewise (not the velvet) the Jacquemart-Andre Museum and the Opera Garnier. For a grimmer interior, the Conciergerie. Wonderful churches -- Notre Dame, Ste-Chappelle, St-Sulpice, St-Germain-des-Pres, Madeleine. Big department stores, and the "passages" -- covered streets full of little shops. Maybe the Catacombs if she doesn't mind tunnels with rows of medieval skulls -- at least it's out of the rain! ;-)

But I agree with others -- in a big city you have lots of ways to duck out of heavy rain, and with the right clothing you can handle drizzle and showers on the streets. Paris is a great walking and people-watching city even in wet weather.

Thanks for the great ideas; especially the horseback riding areas.
I do hope for at least one good day so we can visit Versailles.
Thanks again.

Over in the 5th Arrondissment there is the Manufacture des Gobelins, the Grand Mosque of Paris, and the Arab Institute.

The Manufacture des Gobelins was founded as a dye works in the mid-15th century by Jean Gobelin. In 1662, Louis XIV purchased the Gobelins manufactory and there Colbert united all the royal artisans, creating a royal tapestry and furniture works.
http://www.museums-of-paris.com/musee_en.php?code=349

Yes, it's still a little early for Paris, weather-wise.
But the good news is that Paris has hundreds of things to do indoors, out of the rain.
So just pick anything indoors that you and she enjoy doing.

It never crossed my mind that it was my job to keep my adolescent travelers 'entertained'. We put them in charge of planning some of the trip. My son led me to the Aqueduct Park in Rome, to the John Soanes Museum in London and to Arts et Metiers in Paris and the Specolo in Florence. My daughter took me to the British Library which turned out to my surprise to be fascinating, and up the Tower at Notre Dame and to the Cluny. She also took me to the London Zoo and the wax museum in London both of which were a waste of time -- but it wasn't just my trip, it was hers too.

Seriously give her a good illustrated guidebook and let her make some some plans. It is my and other people's experience that when kids are in charge of other people's happiness as well as their own that everyone has a better time. Paris is a huge city with countless indoor venues and it is also quite lovely in the rain. Rain is mostly episodic and conquered with an umbrella -- but if not on a particular day there is plenty to do inside.

You might consider a trip to the chateau de Chantilly - in addition to the chateau itself there are the Grand Stables and a horse museum. I've never been but it's on my list for a future trip to paris.

for a horsy kid Chantilly is a great idea. And you get a chateau and garden visit as well.