Assuming your friends are still in Paris - I don't know about the boulevard Raspail market, but Batignolles, the other big marche biologique, runs all year round (it's on Saturdays, though).

The Musee Gustave Moreau is one of Paris's oddest, most intriguing and least-visited museums and definitely worth a look; the Atelier Brancusi is less off-the-beaten-path in terms of location (right in front of the Centre Pompidou) but it doesn't get many visitors, which is a real shame. And I think the Musee de la Musique at La Villette has reopened after renovation, but best check first.

I don't know what the weather is like in Paris at the moment, but if it's sunny and not too cold, I'd recommend a wander along the Canal St-Martin with a coffee break at Chez Prune (corner of rue Beaurepaire and quai de Valmy).

Hope that helps - if not for this time, then next time your friends visit!

There is a very nice bookstore called La Hune next to Cafe Flore (see post). Buy yourself a lovely book and sit next door for some hot chocolate. I prefer the upstairs room myself.
I would also suggest going to the Louvre on Friday nights when they are open late. Almost no one goes then and you have vast galleries to yourself until 9:45pm or so. Quite a treat.