Purchasing Train Tickets

All high speed trains in France (TGV) are all reservation only. When all the seats are gone they're gone. Unfortunately for you they are also all sold on the same basis as airline seats - cheap to start and very expensive for walk ups.

So you can likely get from Paris to Avignon or Lyon or Tours on the day, and if one train is sold out (it happens but not all the time) there is another an hour or so later - but it will cost you.

The much slower and much more frequently stopping regional (TER) trains have the same price to walk up as to purchase well in advance, have no seat reservations, and like a city bus you pile on (after validating your ticket, big fines if you don't) and if there isn't a seat you can find then you stand in the corridor or by the door.

Your suggested method of travel isn't how I do it, but with deep pockets and super flexibility it can be done.

Remember too that trains tend to operate with a spoke system with Paris as the center so often it is not easy to get from point A to point B without connecting through Paris or another large city, so plan your route ahead to be efficient. As noted prices are very cheap when you buy tickets 90 days out on high speed trains and many times that initial price if you walk up.

As pointed out, cheaper fares need advance purchase, but going day-by-day isn't ruinously expensive. Right now its Monday evening in Paris. If I decided to go to Lyon tomorrow the website http://en.voyages-sncf.com/en/ shows the TGV price as 75E, so I could go to the nearest train station and buy it tonite. Plugging in a travel date in April I see fares as low as 25E (although not necessarily at a time I'd want, around noon) and many departure times during the day for 30E. So it boils down to how you value spontaneity - is it worth 45E on a leg like this?

I'd add, though, that if you plan on traveling without reserving hotels you need to budget some time for this and accept some inconvenience. The internet is a 2-edged sword. On the one hand it will let you quickly find hotels and call ahead to check for room. The flip side is it lets everyone else do that too. The ease of finding hotels and making reservations in advance with email means more rooms are reserved in advance.

Although Voyage SNCF has tickets for sale at short notice, they can be difficult to purchase online with standard US credit cards. If you have a real chip&PIN card or have Verified by Visa or MasterCard SecureCode, then you have a better chance. Only the cheapest "Prems" rate tickets allow online payment with PayPal, and those will sell out first. Purchasing with a US credit card should be fine at a train station ticket window or SNCF Boutique (ticket office) in other parts of most French cities.

A rail pass does not add flexibility in France, since the number of seat reservations available to pass holders is quite restricted. Don't even consider a pass for the style of travel you're contemplating in France.

For maximum flexibility, you can rent a car for a reserved period and go where you want within that time. Of course a car is not our first choice for big cities, due to traffic and parking issues.