Depending on the length of time you have, there are certain "must-see"
locations in Paris:
These are listed in descending order, since most tourists automatically go to the first four as well as Versailles. [Versailles is outside of Paris and requires a half-day to visit.] In fact, the ones on the second half of the list are best for die-hard tourists who already speak French.
- la Tour Eiffel
- l'Arc de Triomphe
- Notre Dame de Paris
- le Basilique du Sacré-Coeur
- le Louvre et les Jardins des Tuileries
- le Musée d'Orsay
- le Musée Auguste Rodin
- la Madeleine
- le Panthéon
- l'Opéra Garnier
- les Musées du Grand et du Petit Palais
- le Centre Georges Pompidou
- Place de la Concorde (Obélisque de Louqsor)
- les Invalides
- la Conciergerie
- le Palais Royal
- l'Assemblée Nationale
- les Catacombes
- le Tour Montparnasse
Since you probably won't have time to see all of these, you should consider hitting small groups of them which are close to each other.
If I were driving, I would do the Opéra - la Madeleine - Place de la Concorde - Assemblée Nationale - Musée d'Orsay by car (or bus). They're along the same route, so you can pick and choose which ones you want to see.
Another tight grouping which is to start on the Left Bank then go to Notre Dame - Sainte-Chapelle - la Conciergerie - Palais Royal - Louvre. This one can be done on foot, pretty easily.
The Tour Eiffel and the Arc de Triomphe are each further out. If you turn at the Assemblée Nationale and head west (instead of going to the Musée d'Orsay), then you can pick up les Invalides (and the Rodin Museum) and then the Eiffel Tower.
Although the Grand Palais, the Petit Palais, the Centre Pompidou are all admirable museums, if your time is limited you can save them for another trip. Similarly,
Tour Montparnasse and the Catacombs are close to each other, but not much else, so they should probably be left for another time.
Finally, at night, I would choose to go up to Sacré-Coeur. Ride the funicular train -- it's covered by the Mobilis ticket -- and then after seeing the church, cruise around Montmartre. There are several small restaurants and bars with entertainment which can be expensive, so don't eat there unless you can find a cheaper one on a side street.
There is a lot of not very impressive art for sale. Vendors will approach you and try in English to get you to buy it -- don't. Even a lot of the stuff that looks painted (brush strokes and all) is really mass-produced. Also don't stand still too long, someone might clip your silhouette and try to sell it to you for 40-50 FF. It
happened to my companion and me, but the guy was nice and decided to give them to us after we told him we were clergy.
As for the length of time each place takes, I'd count at least an hour off for any place you choose to go in -- except the churches which can usually be fully seen in anywhere from 40 minutes if you're really impressed, down to 10 minutes if they make you sick. Some places take more than an hour. I'd give around an hour for the Eiffel Tower, but that depends on how long you choose to stay.
The Musée d'Orsay should take under two hours, the Louvre can be done under three hours (I've done it in about 2 1/2, setting a land speed record in the process), but probably should take a minimum of four hours.
Transportation between sites can be quick (5-10 minutes) by Métro (subway), though by car, you shouldn't try to cram too much in, because traffic can get really tied up in rush hour.
Before you go, you can download Métro map (114 K) here.