I love going to France... but lately have been going to the South of France to visit friends, rather than Paris. When you are in Paris, find the spot to buy the Museum pass as there are several you want to visit (others have mentioned them). The Metro is so simple to figure out... my kids would do the navigation when they were quite young, you will have no problem. You do not need a car in Paris... more hassle than worth. Also, make sure you go to Chteau de Versailles... it is magnificent and you can walk forever through the gardens.
I usually stay in smaller places where I can experience more of the culture. I have found the French to be wonderful people... if you do not know french, try to learn a few key phrases before you go as it will be helpful.