If you will be in Paris on a Saturday or Sunday, I highly recommend, HIGHLY recommend, going to the Marche aux Puces at St Ouen. They are "flea markets" but more like antique markets, crafts fairs, etc...everyone we ever took to that place had a delightful time. I found the most amazing fashion magazines from the late 19th century with beautiful prints PERFECT for framing. There really is something for everyone. The place is HUGE! The best metro stop is Porte de Clingancourt on Metro line 4. Make sure to have a map! You will need to walk toward the freeway and go under the underpass. There is a true flea market before the underpass and a few people have mistaken that as the place. Don't make that mistake. (I did this and LOVED it. Found some great little treasures, but if you aren't interested in antiques, don't bother with it)
The Marais neighborhood is also a MUST for boutique shopping and for just soaking up the environment. This is one of my favorite neighborhoods and worthy of some time wandering around. If you are interested in a walking tour, check out http://www.paris-walks.com/ the Marais and Montmartre walks are great, and take you to neighborhoods you will LOVE. We really recommend these. There are Marais and Montmartre tours on Saturday and Sunday. Check their schedule, you don't have to reserve in advance, you just show up at the meeting place. Really easy, really great. (DEFINITELY, definitely go on a "paris-walk". They are a great way to get to know the neighborhoods and hear all the interesting stories and history of the place. The Marais was easily my favorite neighborhood.)
You have to, have to, HAVE to eat a macaroon from LaDuree. They aren't macaroons like Americans think of, they are something totally different, not coconut. Also, the palmier cookies are quite nice (but only get them if you are also getting macaroons). The raspberry, pistachio and the salty caramel flavors are our personal faves. There are two locations in Paris, one on the Champs Elysees and the other on the corner of rue Royale and Faubourg-St Honore (this is often the best and easiest to get to-- just by the Madeleine and Place de la Bastille). The lines later in the day can be long, but move and are worth it. If you want to eat there, you don't have to wait in line-- just go in, there is a hostess on the right as you enter. Don't be shy, only tourists wait in line if they are eating inside. You can order a meal, or just a drink and the macaroons. (We went back to LaDuree three times I think. We couldn't get enough of the macaroons OR the ambience.)
Window of La Duree
Look for the "Le Routard" sticker on restaurant windows. This is like Zagats, but for the backpacker. It is less expensive, but divine. Truly, I have never gone wrong in a Le Routard recommended restaurant or shop. I attached a picture of what he looks like so you will know when you are out and about. We even ate at these places in the south, and they were awesome! (You'll see these on the windows but you can also look them up online or buy the book. It's a great way to eat well but cheaply in France.)
Falafels in the Marais. If you are already in the neighborhood especially, They are on rue de Rosiers (if you do the tour, you should go right by this).
Falafels in the Marais
Crepes! Try to find a creperie where you can sit down and eat. The choices there will be awesome. Try a salty caramel one if you get a chance, you won't be disappointed.
MUSEUMS: If you must do the Louvre, consider doing it on a night when it is open until 10pm (Wednesday and Fridays). If you have the Rick Steve's guide, his Louvre tour is the best (perfect amount of time and the real highlights), unless of course you were an art history major, you won't spend more than 1.5 hours there. If you must chose, go to the Musee d'Orsay, the top floor has the impressionists and most of the art work that many of us really enjoy looking at. I personally think the Orangerie, Rodin, and Picasso museums are better than the larger more popular ones, but with only three days, most people want to hit the big names.-Consider buying the Museum Pass, it will get you into almost all of the big ticket places, including Versailles, but not the Eiffel Tower. Check it out at: http://www.parismuseumpass.com/
VERSAILLES -- If you make a trip to Versailles, go early! It will take up most of your day. After touring the palace, consider renting a golf cart to cruise around the grounds. They are enormous, and you will be cold. You must maximize your time. You can rent them just after you enter the gardens before you begin to go down hill. Again, you will be cold. Take lots of layers that day, don't forget your umbrella, hat, scarf, and your most comfy walking shoes (I say this because we had 36 visitors and none of them ever were dressed comfortably there!). Try and get to Marie Antoinette's Hamlet (Le Petite Hameau). I think it is the best part!
METRO -- Buy a carnet on the metro (said car-nay). It is a set of ten tickets, cheaper than if you buy individually and you don't have to stand in line to buy a ticket every time you get on again.
THE MARAIS -- I recommend doing the Marais on Sunday since it is the Jewish quarter, almost everything is open there, where other neighborhoods tend to be a bit dead. Church is also located in the Marais.
NOTRE-DAME -- Go to Notre Dame on Sunday morning because they have church service you can attend. Fascinating. I just happened upon it and then went to the Paris ward from there since it's not too far on the metro.