For a trip of this length where you can afford to be quite flexible, I like acraven's suggested approach. I would make a list of categories of things you want to see/do and place them in categories with all the relevant logistic information (e.g. museums, outdoor spaces/parks, historic sites, day-trips, shopping/stores, outdoor markets, cocktail bars, etc). I also second the suggestion to research if the day trips can be done with local regional transportation whose price never changes, or are TGV-type destinations.
Unfortunately, the cheap tickets are not refundable; I think they are
now changeable for a fee plus whatever the additional cost of current
tickets (but I have not verified this).
In a very rare change for these days, the current policy simplified in May 2019 actually generally made things more flexible. The vast majority of TGV second-class tickets (aside from Ouigo), even cheap advance-purchase tickets bought many months in advance, now have the following policy:
Billet échangeable (ajustement au tarif en vigueur) et remboursable
uniquement avant départ : 5 € de frais dès 30 jours avant départ, puis
15 € 2 jours avant départ. Dès 30 min avant départ, billet échangeable
2 fois max (même jour, même trajet) et non remboursable après 1
Ticket is exchangeable (fare differences will apply) and refundable
only before departure: €5 fee starting from 30 days prior to
departure, then €15 fee starting 2 days prior to departure. Starting
from 30 minutes prior to departure, the ticket can be exchanged a
maximum of 2 times (for the same day and the same journey) and is not
refundable after 1 exchange.
Ouigo trains have their own much more restrictive policy, but in my experience the vast majority of "normal" TGV tickets now have this policy (I check on oui.sncf where the policies are stated very clearly for each ticket).