Paris for two days

Stella's given you the top things IMO. I'd totally skip La Defense with as little time as you have. With just two days, I'd recommend a HO/HO bus to get a good above ground tour. I've done Les Cars Rouges and recommend them.

I would not leave without: 1. taking the elevator up to 2nd level of the Eiffel Tower, any time of day when visibility is good. Takes time but this experience is really fun. And be sure to see the tower "light show" after dark, from some close by point, somtime during your stay. 2. Visiting Notre Dame & walking the Ile Ste Louis and la Cite & Seine river banks.
2.5 experiencing a Parisian cafe for coffee, drinks or a meal at at least one nice place. If I had extra time, I'd chose from these: 3. See the Louvre pyramid, stroll from there through the Tuilleries to Place de Concorde. 4. See the Arch de Triomphe. 5. Try to see some of the art in the D'Orsay museum, if time permits (museum going is time consuming). 6. Stroll the Marais, Palais Royale or Place des Vosges - shop or people-watch from a cafe or park bench.

Hello Catherine - That's not much time - but I would recommend of course the Eiffel Tower, the Louvre, Notre Dame & a Seine River cruise at dusk. After that I would just enjoy Paris - walk around - hang out at Luxembourg Gardens and people watch.
Enjoy! Stella

Get an RER to the Grand Arche in La Defense and then walk down to the Champs-Élysées, across the Seine and then as far toward the centre as you can manage.
Roger

In addition to Stella's suggestion to hang out in the Luxembourg Gardens (one of my favorite places in Paris), I would suggest walking in and through Ile St. Louis (little island to the east of Notre Dame) and Ile de la Cite (island where Notre Dame is), then along the river.

Get a copy of RS Paris guide book and read it as it has lots of good information for you.

This (and all of its cousins) is a duplicate post from General Europe, in case you thought you were seeing things.

I would do a bike tour or a walking tour to get the most out of your time there. As to what to see, what is it that you have always wanted to see in Paris? What are your interests? Why would you want to go look at the same stuff that I am interested in? I enjoy old architecture and old churches and military history. Do pick out sites that YOU want, not someone elses ideas. Though I wouldn't mind going in some art museums, they weren't at the top of my list, so didn't go in any while I was in Paris. For me, the highlight was walking to the top of the dome in the Sacre' Coeur, being able to look out over the city, and actually almost look down onto the Eiffel Tower.

Take a look at the Fat Tire tours - they have both bicycle tours and Segway tours. A bit pricey but quite good.