Taking the advise of my kind fellow travellers We have decided to take handbook and map in hand and tour Paris on our own.
I am wondering what you think of my three day itinerary and if I should reconsider anything. We are four sisters in our 50s.
Day 1 (Sunday)
We will be arriving in CDG airport ( from Canada) at 10:30 am and will head to our hotel in the Latin Quarter. We'll rest up until about 1pm then walk to the Louvre, it's close and spend approximately 3 hours there.
We'll then go to Notre Dame cathedral and on our way back stop at the St. Sulpice church.
We'll have a light supper somewhere then go to bed at a reasonable hour.
Day2 (Monday)
First of all, we'll head to the Marche aux Puces de St Ouen from 9 until about noon. (One of our group is an avid antique collector.) Then we'll head to the Montmartre area for lunch, exploring, and to check out Sacre Coeur. We'll head back to the Latin Quarter about 4 pm, stroll a bit through the Luxembourg gardens, and then check out the Pantheon before having a late supper .
Day 3 (Tuesday)
Take an early metro to Versailles for 9 am and spend until noon there.
Head back to central Paris to check out the Invalids, and to take a boat tour down the Seine. Then we want to walk along the Champs Elyees to the Arc de Triomphe and visit the Eiffel Tower. And if time, take a 2 hour city tour to check out on what we missed. Are there boat tours where we can get off at the other end?
Day 4
We're leaving really early and heading to CDG airport for the next part of our trip.
Well my friends, is this too much? What do you think?

Uh, I'll just tackle Tuesday and let others address the things they'll know better.
What you're covering just on Tuesday took us three full days, and that was being skimpy on much of it... we didn't go up the Eiffel Tower or the Arc de Triomphe (er, blame my heights issues for that, partly).
Three hours at Versailles might be enough time for one wing, if you have a museum pass and can skip the entrance line. The train ride is about an hour each way, so there's five hours of your day (and you won't see much of anything in that time). Maybe if you cut Versailles from Tuesday you could do everything else you've listed, but it would be really rushed and I really doubt there would be time for either the "2 hour city tour" or... eating?
Also, depending on when this is planned, be aware that it can get really hot in Paris. When we were there it was hitting 95F, and more than an hour of walking around was super tiring. Versailles isn't air conditioned, and though the gardens are lovely there's not much shade outside.
I have seen worse itineraries in terms of cramming things into a single day and in terms of time management (12.15 lunch. 12 45 leave for walk through park. 13.15 walk to louvre...etc) I think your third day is the most challenging one- after versailles you probably want to take a break rather than a boat, a city tour, a walk down teh champs and a stop at Invalides.
So i don;t think it's outrageous and i think it's doable. But.... i think that be planning everything in advance you will miss the opportunity to discover some places for your self, and spend a little more time in a neighbourhood you like.. it is not my style of travel but to each his own.
#1, I am shocked to learn that Versailles is not airconditioned. You would have thought Louis XIV could have done better than that.
You'd be surprised what people expect, eh? I heard tons of people complaining while we were there.
I suppose we were used to it, as our flat doesn't have a/c either. ;)

I don't think it is too crowded but a few points may help. Arriving CDG at 1030 means that it will be close to 1230 by the time you arrive at your hotel and you may not be able to get into your room until 1400 or 1500 though you will probably be able to leave your bags. Hitting the Louvre straight away at 1300 is not a good idea, especially if it is the first Sunday of the month. For a first visit to the Louvre, you need all your wits about you: better to do it when you are fresher. Besides it will be much better for you to walk around in the open air for a couple of hours. I suggest rearranging things a bit. and do part of your Tuesday afternoon here. Your ticket to the Louvre allows multiple entries on the same day so you could go there first thing on Mon morning for an hour, then off to the market and back to the Louvre for another hour or so. Notre Dame on Sunday afternoon, depends on how late in the afternoon it is. ND has Mass at 1800 so if you want to be able to walk the whole cathedral you would need to be there by 5.00pm. Close to the Mass time you can still get in but certain areas are cordoned off. While you are in the area, St Severin is worth a look.
Enjoy yourselves
You MUST see Sainte Chapelle, one street west of Notre Dame and IMHO better than ND. Do not forget to go upstairs into the upper chapel and experience the pink hue from the sun shining thru the stained glass windows. It would easily fit into the day you visit ND. SeeS ainte Chapelle at - http://en.wikipedia.org/wiki/Sainte-Chapelle
After all the fountains, didn't have enough left for AC. If the people didn't eat so much cake, maybe it could have been done.
Besure to get to Versailles before they open and be sure to be the first one in line. The keep an eye on the time.
I also agree that you should see St. Chapelle.

Just came back from Paris. June was cool and a bit of rain. Our apart had no A/C but did not need it. The advise given so far is good.
Everything that you plan is doable and we did it but at a slower pace, we had a month . ..you will be busy but it is possible. Loved all of Paris but we recommend skipping the Pantheon. Not worth what is contains nor the 9.0 Euro entrance fee.
Do not know about the on-off boat tours. The boat tours we took were round trip starting two bridges upstream from the Eiffle tower.
The Dorsay museum is pricesless. Its main restaurant is first class for 35 euro lunch w/o drinks.
We had no waiting line at Versaille. Make sure you know which terminal at CDG you are departing from...to tell the taxi driver.

also just returned from a fortnight in Paris. Compared to London it was very dirty and hideously expensive. Still a very interesting place to visit though. I think your itenary is way too busy. Slow down and you will enjoy it a lot more. No point rushing around the place trying to see everything at once.