Any trip to Paris should immediately begin with L'As du Fallafel in the Jewish Quarter of the 4th arrondissement. The rest of your first day should be a preparation against adult-onset jet lag.
One thing about Paris that people always love to tell you, but I personally have never, ever, ever, ever experienced, is the classic line: Parisians are rude to Americans. I don't think Parisians are inherently rude to anyone. They're just not interested in acting like you're cool or special because you're an American in Paris on vacation. Therefore, when you walk in all wide-eyed, tell them your little American joke, and try to say something clever in French, they roll their eyes and walk the other way. But, hell, I do that to other Americans in America!
Parisians are very interested in America, from what I've seen. Everywhere you go there are American-style 1950s rock and roll diners. We went to a café just to get a cup of coffee and found ourselves in the middle of Buddy Holly hour. Just about every boutique is pumping out Coltraine or Sinatra. There are cheeseburgers everywhere; even at the classic bistros, not just the tourist traps. We finished our first day by drinking cocktails in a New York-inspired bar. If there's any one thing staring you dead in the face about Paris, it's that they love American things.
Maybe a better way to put that old adage is this: Parisians won't validate your ego when you're fishing for attention. Americans love to make their presence known in just about any situation, but that doesn't fly over here no matter where you're from. Focus on yourself, mind your own business, be polite, start by saying Bon Jour, at least try to start the conversation in French, and you should be just fine. I had nothing but friendly conversations with cab drivers, waiters, bartenders, and shop owners all day long.