If you travel by plane and/or train, you will probably need to hire a car. There are plenty of hire companies at all the main airports and train stations. It may pay to shop around beforehand on the internet: prices can vary enormously.
If you do hire beforehand, remember to factor in any travel time by train, debarkation time for planes and potential delays. Most hire sites (outside Paris) close about 6pm but they may make arangements for you to pick up a car after hours, if you ask.