If you plan a relatively short stay in France or Western Europe, perhaps a rental car is the best way to get around. All the major US car rental companies have agencies in Europe, usually with pickup/drop-off points at major airports. A couple to look at would be avis.com and hertz.com. But, the internet will show you many more.
For longer stays in France and Europe (more than 21 days), a lease program will prove to be the most economical way to go. Citroen, Peugeot and Renault offer very good lease programs for extended stays and have their own websites. Yerda and I have used Europe-by-Car (ebc.com) for many, many years and have had wonderful satisfaction. They supply new cars with complete insurance and maintenance coverage at very reasonable rates.
Train travel in France is excellent and Eurail passes are available to North Americans (they must be purchased before you leave North America). Otherwise, specific destination tickets can be purchased at any station on the spur of the moment. For those of you who will be visiting us this summer, we will give you the name of the closest station (and the Paris departure station) to the boat. We will gladly help you with your planning and will meet your train when you arrive. The French National Rail System website for fares and schedules can be found at sncf.com.
Bus service in France appears to be limited to local travel between towns and villages. It is not likely that you will find bus travel a viable means of getting from A to B unless it is on sightseeing jaunts.