I agree with many of these suggestions to manage your time well - and also thought a 2 day Museum Pass might be helpful. I like the freedom it gives you to pop in to places for shorter visits if you're nearby - and you might see some things you otherwise might not choose - like the Cluny, for example. It is interesting and small, and we didn't see it until our 3rd trip...and then went there again with our kids last summer. They liked it too, (2 college-aged "kids" along with a teen) - so that is something to consider. The Museum Pass would cover most of what you've identified as places you're like to see.
Personally, I saw enough of the Champs-Elysées from the top of the Arc de Triomphe - and liked that perspective better! You can really appreciate the layout of the streets and buildings from that view, and being there at nightfall is especially nice. I found the avenue at ground level to be loud, smoggy, crowded, and very commercial. That's a matter of personal taste, of course - but I would agree with another poster that finding a section of Paris to browse and explore might give you a better feel of the city, rather than following a frantic point-to-point itinerary.
Our family stayed in the Latin Quarter for part of our visit simply because there were so many things to see and do there that would be interesting to our kids (and we had Museum Passes). Fun and varied shops to explore, Marks and Spencer for yummy picnic items, the nearby Luxembourg Gardens for picnics and strolling, close to the Cluny, the Pantheon, and more. Very close to the Seine as well - a great central location for our explorations. We really liked being able to find a great breakfast cafe - and then return there the next day and the next, as we familiarized ourselves with "our neighborhood" for our stay. There are a lot of places to eat in that area, for a variety of prices. I think it would be a good area for college students to explore.
I've heard that the free days at the museums are really crowded - and I must say that there is a lot of pleasure for me to bypass a line with my trusty Museum Pass :0) The Musee d'Orsay is absolutely wonderful - and more manageable in the evening it if is crowded. We went to the Louvre one very long and hot day last summer - spent about 6 hours there, went back to the hotel for more AC and resting, and then returned in the evening, as it was a late night. I loved being able to do that with the pass as well - so much less crowded, and easier to take our time enjoying the art in the evening.
As for both Paris and Florence, you might enjoy downloading Rick's audio tours to listen to either before you get there, or as you walk through some of the sights. I learned a lot more about the collection at the Accademia beyond Michelangelo's David by listening to the audio before and during my visit there. There was a great shop for picnic items right outside the Accademia Gallery - reasonable prices for the students in our tour. A lot of the fun for us in our travels was checking out the food stores and finding new things to try - we didn't want to do the restaurant scene all the time as it would have been really expensive for the five of us!
Check out the websites for the Orsay and Louvre - it may help you plan your visit if you know you'll have limited time. Hopefully you can go with the attitude that you'll be back to see more next time! Paris has so much to offer...I hope I'll have another chance to return.
Enjoy your trip!
Laurie
