Beware of private limo drivers approaching you at the airport
They will tell you they charge the same as a regular taxi driver does, but it is not true. They charge more, and it is not legal. It's not dangerous, per se, just not a good idea... Some of them are private limo drivers, others are simply plain people with no taxi license whatsoever and often no particular taxi insurance... Go to the taxi line like everybody else :-)
Avoid restaurant patios unless you like cigarette smoke
This one is sad but true. Since the 2007 law which forbids smoking inside restaurants, all the smokers go to "la terrasse,"which is outside and therefore is excluded from the law. Smokers now rule the French patios, so avoid sitting there if you can't stand the smell of cigarette smoke.
Don't plan to go on museums on Mondays or Tuesdays
Many Parisian monuments close on either Monday or Tuesday, and on holidays, so make sure you check their opening days before you show up. You will find much more practical Paris info in my French Audio Books, in particular Une Semaine à Paris.
Traveling in the subway with suitcases, strollers...
Or be prepared to face long walks and stairs... especially if you need to connect between lines. There is no telling on the "plan de métro" how long a connection is: some take 15 minutes of fast-paced walking with at least 6 flights of stairs! Try that with a large suitcase... Or rather, don't - I did it for you! It's painful. Which brings us to the next piece of advice... Find more about the Paris métro.
Don't take large suitcases to Paris
Compared to the US, things are smaller in Paris: car trunks, elevators... It's actually impressive how tiny the elevators are!! So pack smart, and think small... (Same goes for baby strollers: it'sclose to impossible to fit an American-sized one into a French rental car.)
Getting a taxi can be very tricky
If you are staying at a hotel, the clerk should be able to call one up/ schedule one for you. There are also taxi lines where you can wait and get a taxi - if you are lucky, that is... Like anywhere else, taxis get super busy on rainy weekend nights..... so if you need to go somewhere, there are numbers you can call: les taxis Parisiens, les taxis G7... Read my article on How to get a taxi in Paris.
Don't holler at your wait person.
All you have to do is to catch their eyes, maybe do a discreet hand gesture, and they'll get to you as soon as they can. Don't call your waiter "garçon" either, it's only applicable to a very specific type of waiter "le garçon de café" and is very old-fashioned anyway... Use Monsieur, Madame or Mademoiselle. Learn more about French politeness in my French audio lesson.
Beware of "Do you speak English" scams
Paris is a big city and relatively safe. Like in any other big city, you might run into pickpockets, beggars of all sorts, and of course scammers. Some will start their spiel with "Do you speak English?" then be all nice and friendly, tell you their oh-so-sad story, and eventually ask for money. Another popular scam is the "You dropped something!" and then the person hands you what looks like a valuable piece of jewelry... Particularly effective in front of a jewelry store :-) While you are busy talking, a partner visits your bag or the person accuses you of robbing them, all ending up in asking for money. Just be aware of your surroundings.
Some areas of Paris are not safe or family friendly
Certain parts of the Rue St. Denis, a few blocks from the Centre Pompidou can be a bit dicey. There's a fair number of prostitutes and junkies around there. Ditto for the side streets around the Place Pigalle down the hill from Montmartre (up the hill is fine). Neither place should worry an NY'er very much, but they are certainly not family appropriate. You should also avoid walking at nighttime on the Barbes Boulevard, the Forum des halles and the Seine banks (by the river; up the street is fine). Most Paris suburbs are safe enough, but don't go to St Denis, La Courneuve, or Mantes-la-Jolie unless you want to see what a French ghetto is like.