Jim, I rented a Paris apartment through VRBO.com for my upcoming vacation on advice from my sister who stayed in the same apartment each summer for the last 2 years. The location of where you will stay is very important. I like staying in the 6eme arrondisement (on the Left Bank) which is wonderful for restaurants, outdoor cafes, shopping, people watching and public gardens. Everything will be just outside my door!
My favorite Paris activities are:
*Strolling around the Marais district to window shop. Eat at a restaurant within the Place de Vosges
*Enjoying some relaxation time and people watching in the Luxembourg Gardens. Bring a picnic.
*Having drinks at a cafe in the shadow of Notre Dame Cathedral at night
*Visiting the Musee Jacquemart Andre with stunning interiors and art
*Browse along the Ile Ste-Louis and eat ice cream at Berthillon (glace et sorbet)
Best thing to do in Paris is to just be outside to soak up the atmosphere.
On a previous trip (many years ago), I stayed in Tours which I believe is on the TGV line. Anyhow, I signed up for a chateaux mini-van excursion and the tour van met up with some folks who arrived from Paris just to do the excursion. Seeing the variety of chateaux in the Loire Valley: some of them lived in homes, some of them with beautiful gardens, some of them immense vacant properties, you realize you're in France! Only 1.5 hours or so from Paris. And as a plus, this is wine country...the land of Vouvray and other wines.
Once your wife and mother-in-law leave and you have your solo time, you can go to town! I have visited all the places you mentioned in Italy. Love Rome! Love CT! My advice: less is more. Take the time to relax and notice the little things around you and keep la dolce vita in mind. Enjoy Italy and don't rush. Check out the Italy forum for information (but I found there are cheap flights from Paris to Turin, Italy).