I know that you posted your question earlier in the month but I just got back from a week in Paris and thought I would mention something that I didn't see anyone else mention. There are 2 major bus tour companies in Paris. One is red and is sponsored by a private company. The other is part of the greater Paris metro system. The bright green "L'Open Air" Tour buses are double decker buses which have guided tours to all different parts of the city. You get on the bus, pay at that point for the day (credit cards accepted on the bus!), you are given your earphones and then as the bus drives around the city, you can hear info as well as see them. You can either be on the top in the open air, which has the best view, or inside on the first floor of the bus. It has a get on/get off option, so you can take the bus, and if you see something you like you can get off the bus and sight see. Then, when you are done, get back on the next bus, which come about every 20 minutes. The green line is the one that hits most of the major sights. I know it sounds dumb to take a bus tour, but we found it was a great way to see what was out there and then to decide where we wanted to return. We actually bought a 2 day pass, and the first day we enjoyed it just to see the city, then the 2nd day we used it as a taxi service :), getting on & off. The metro is quick and easy to understand, but it IS underground, so you really don't see anything. Here is some info about the bus, and the link is in here too:
I hope you have a wonderful time!
PS If you can possibly see the Paris Opera House, you will never forget it.