We are just wrapping up Day 5 of 6 full days in Paris. Today we went to Versailles: a very full and tiring day! We have seen a lot in Paris besides Versailles. In 4 days we saw: The Louvre, the Picasso Museum, Notre Dame & Saint Chappelle among other sites on Rick Steves' Historic Paris Walk, the Marais, Sacre Coure & Montmarte, R.S Place de Madeleine Shopping Walk and Galleries Lafayette, the Eiffel Tower, Arc di Triomphe and Champs-Elysees, the Orangerie Museum, and a long walk through an elegant neighborhood we "live" in.
Yet to come tomorrow are The Musee D'Orsay and R.S. Left Bank Walk. Whew! We are still missing a lot: The Pompidou Center, Rodin Museum, the Marmottan Museum, Bateaux Mouches we intended to ride, Rue Cler, etc. Wanted to do more shopping but we are out of time! And we take time to relax: a 2-3 hour break in the afternoon so we can regroup for the next assault and to refresh ourselves. I think in 8 days one trip out-of-Paris-center will be enough. I suggest you plot out what you want to do in 1/2 day increments. What will you do in the morning and what in the afternoon/early evening? What is weather dependent and should be moved around as the forecast reveals itself? When we have a long stay in one location, I find I can sort of plug-and-play my itinerary formed this way, choosing based on weather, energy, crowds, our mood, or taking advantage of something that comes up that I did not know about before traveling.
BTW, the trains are great as are the Metro and buses, but it takes some route planning. This site, RATP, has been very helpful in planning our transit.
Have you considered an apartment for two couples? We like having a little kitchen, a place to make coffee, occasionally a light lunch "in", and a place to have a glass of wine and relax between events.