Hello Mary S,
I'm jealous of the amount of time you get to spend on your trip and I would second (or third) the idea of perhaps seeing more locations. You mentioned sketching - if art is an area of interest I might make the following suggestions.
In Paris aside from the obvious big museums there is also:
Rodin Museum: Sculpture museum set in his old house in Paris with rose garden
Picasso Museum: Paintings, sculpture and ceramics set in a villa
Marmottan Museum: Monet museum and you can also visit one of his muse gardens at Giverny
Luxembourg Park: If you are looking for a quiet spot in the city this giant park is always busy but also always has some unused section
And while I haven't been (yet) every art lover who has visited L'Atelier des Lumières raves about it. (I'm going in Paris and and plan on going in Les Baux as well.)
https://www.atelier-lumieres.com/
Lyon would make a great stop - you're going through it on the train anyway and while it is a big city there is plenty to see and do. A great old town to explore and the Musee des Beaux Arts. (Also Great Roman theater ruins above the city.)
You'll also be passing right through Provence.
If you stop in Arles you can see where van Gogh painted and they have metal "easels" set up where you can compare the painting with the current view. (More great Roman ruins in Arles and a good Ancient History Museum.)
Avignon is pretty, charming town although most of the art museums are smaller personal collection style.
There are a ton of tiny charming, quiet hill towns in Provence - especially in the Luberon hill towns but getting to those on public transportation can limit your options. But Isle-sur-la-Sorgue should be reachable by bus if that interests you.
On the coast don't miss Antibes and the Picasso Museum set on coast there.
I've never been but Aix-en-Provence has many art sights including Cézanne’s house and art studio.
In Nice don't miss the amazing Chagall Museum hanging just above the Roman Bath sight there.
I can't recommend a really good guide book(s) and research before you go to maximize seeing what you want. There are a lot of little art centric sights scattered on the Riviera and you'll have time to see as many as you like. Search Art Trails of the Riviera for more guides and site suggestions. (I think the Pilot one is a good start: http://www.pilotguides.com/articles/art-trails-of-the-french-riviera/ ) But you'll need to tailor any suggestions to your interests and, schedule, pace and accessibility.
Have a great trip!
=Tod