Laurie,
Lyon is a 6 hour drive from Tours (an example of a Loire Valley location). From Amboise about 5 hours. How long will you be in France? Were you thinking that Lyons and the Loire Valley were perhaps closer together? Forgive me if I misinterpreted your question.
If you are just considering Paris and the Loire Valley, as per the title, I suggest flying into Paris, taking a train to the Loire Valley (St. Pierre de Corps e.g.) and renting a car there. Return the car in the Loire and take a train back to Paris for the latter part of your trip. Driving in Paris is not for the faint-hearted, plus parking is expensive and a car is unnecessary there.
If you are adding Lyon to this trip as well, I have no advice to give, having never stayed there, but I would take a train there after seeing the Loire Valley. There is a fast TGV from Paris that takes about 2 hours. You don't need a car in Lyon if you are staying just in the city. Give us an idea of how much time you have. I would go from CDG to the Loire (e.g. Tours), have a car there for ? days, return it and take the train back to Paris. If going to Lyon, change train stations and take a TGV to Lyon, stay there for ? days and TGV back to Paris to see that wonderful city. Taxi to CDG for return. If no Lyon, then drop that portion.
Have a fun trip!