Other than of course the plethora of museums, does anyone have any ideas for things to do on a rainy day in Paris?

Rain or not, find a nice cafe, order a carafe of wine and some cheese (or coffee and croissant, depending on the time of day), bring a book, and enjoy a slow quiet afternoon

If you speak French, go to the movies. Or go to a grocery store and marvel at all the wonderful things to eat. Go to Gibert-Jeune. Take any bus to the end of the line, then back again. Go up the Tour Montparnasse and have a coffee or drink and see the city spread out below you. Go play at the Cité de Science.

Visit the department stores - there are several.

Or the Grande Galerie de l'Evolution at the Jardin des Plantes. Visit the Quatre Temps shopping mall at la Défense. Go to the Grévin wax museum and visit all of the old covered passages in the center of Paris. Catacombs.
Etc.

However, with a good umbrella, I know several people who love to walk through Paris in the rain with shiny cobblestones and and wet zinc roofs.

Check the weather map and take the train to Nancy, Strasbourg or Dijon.

Thanks to you all! Now I'm hoping for a rainy day or two!

Explore the covered shopping passages. Check out this link:

http://www.answers.com/topic/covered-passages

Check out the Carousel de Louvre ...kind of a Mall under the Louvre.

If you've never been to the Science Museum don't miss it.

Hey Kerouac, I've happily gone for a rainy walk in Paris and in Rome too. If you are properly dressed and wearing good shoes it's a lovely thing to do!! A great suggestion as always.

Have afternoon tea at the Mariage Frères shop in the Marais, then browse the huge assortment of teas.

You can go to the movies without speaking French. Some theaters show movies in the origical language (marked VO in the ads) with French subtitles. And there are often film festivals with movies in original languages.

But - I would still hed for a musuem and lave the movies for evening.

I feel obliged to mention that there are some people who have gone to the movies in Paris and loved it and who suddenly start thinking that "VO" means that the movie is in English. It can easily be in Japanese, Spanish, Italian or any other language... Also there are some bilingual movies, like "2 Days in Paris" from a couple of years ago -- don't forget that the parts in English will have French subtitles and the parts in French won't have any subtitles at all.