Salut ! Back from Loire Valley this week, and I took a couple of trips out of Tours that would also be doable from Paris via SNCF: Chartres and Bordeaux. Chartres is probably already on your radar as part of your Loire Valley séjourn, but the train station is centrally located to the centre ville and an easy walk to the cathedral. The same is true for Bordeaux also, although that's a bit longer of a trip from Paris (3.5 hrs).
In either case, definitely get a chip-based credit card if you don't already have one. Some SNCF stations are manned only (like Amboise), but some are ticketing machine only (like Tours) and you will need a chip card to buy tickets (it doesn't have to be chip and pin).