TGV accessible towns around where I live :
Montpellier is lovely but is currently undergoing some major redevelopment and there is a lot of building work in the centre making it tricky to get around unless you really know the transport system (there's an app which is brilliant though). There are loads of things to do (using public transport) such as a free zoo (Lunaret), an aquarium, an ice rink, cinema etc (down at Odysseum, lots of shops there too). You can even get the tram and bus down to the beach.
Avignon is stunning and really touristic (in a good sense), there's lots to do within walking distance of the central train station as everything is quite condensed (you want to stay in or close to the ramparts). It's much much smaller than Montpellier, Lyon and Marseilles, but just as interesting for a short holiday.
Nîmes is OK, but not at all aimed at tourists, there are few interesting things to do (Roman based).
You can get busses to the Pont du Gard (On the 5€ note) from Nîmes and Avignon which is worth the detour.
Lyon is nice (just don't go on first impressions when you leave the train station, its a bit grim). It's not a city I've spent much time in so can't really comment, same for
Marseilles which is pretty but I personally don't feel very safe when I go there.