Great thread! I actually had a similar trip to Paris with my mom in January. (She's 60, I'm 26...we also stayed for a 4-day weekend!) We took advantage of the Air France special and it included hotel (Hotel Duminy Vendome in the 1st). Definitely a good deal! We did the Carte Musees also, which we thought was great. I'd been to Paris before, a couple of times, but it was my mother's first time.

Other than the stuff already talked about above (major museums, bateaux mouches, etc.), we particularly enjoyed walking around the Marais. Don't miss Place des Vosges. The Picasso museum and Rodin museum were quite interesting. Also the Bastille had a huge outdoor market on Sunday that was really fun to walk through. Shopping (mostly window shopping) was a great experience -- the grands magasins are such a unique Paris experience.

Even though we were there in January, the weather was quite nice and good for walking around. I see the OP is from New York, and you probably won't find the bitter cold feels-like-minus-20 weather of the recent east coast winters in Paris unless you're really really unlucky. We thought it was really warm in Paris the week we were there, in comparison to Boston (even though it was cold enough to snow! Go figure.)

If you splurge on a fancy dinner one night, I'd highly recommend Georges in the Pompidou Center. We had a great table with a view of the Eiffel tower!

If you have any other questions feel free to PM me.