Appreciate input for 3-week France trip planning

About your time in Nice: There's a great deal of art, especially modern art, down in that part of France. Nice alone has at least six art museums, and many of the smaller towns have worthwhile art-related sights (Fondation Maeght in St-Paul-de-Vence, the Leger Museum in Biot and the Picasso Museum in Antibes are just a few of them), plus there's the whole "being there" thing, and the markets, and the gardens... Four days is not very long for that area at all. It would frustrate me greatly. Honestly, I'd recommend that you focus your time either in the north or in the south rather than trying to cover both areas.

In terms of less-touristy places in the south, I enjoyed wandering around Vence, which is accessible by bus from Nice and from St-Paul-de-Vence. While SPdV is shoulder-to-shoulder tourists, I was about the only foreigner in the historic district of Vence. While I was there I chatted with a local woman who very highly recommended Tourrettes-sur-Loup as a beautiful hill town that just doesn't get much tourist traffic. I didn't have time to go there, and I'm not sure whether you can get all the way there by public bus. Perhaps it's possible in high season.

In general you can expect any town with train service to have tons of tourists. The places accessible only by bus are often not as crowded (St-Paul-de-Vence being a major exception). However, with the short length of your visit, you may find trekking to multiple bus-only destinations too time-consuming. I think looking for a one-day bus tour that will get you to several of the smaller places isn't a bad idea. However, as far as St-Paul-de-Vence is concerned, you'll probably want more time there than a bus tour will allow, because of the Fondation Maeght.

I enjoyed Nancy a lot. As of 2017 the tourist office had a handy brochure identifying a lot of Art Nouveau buildings in the city. I had a great time tracking them down on foot and was pleasantly surprised that I encountered others (plus some Art Deco) as I looked for the listed buildings. The Ecole de Nancy Museum is great for Art Nouveau decorative arts, and the city's Museum of Fine Arts is also good; I believe it has some Art Nouveau glass on display. There's also the Museum of Lorraine, but I'm totally blanking on what I saw there.

I also loved Colmar, touristy though it was. Be sure to see the Unterlinden Museum. It's a good one, even aside from the Isenheim Altarpiece. For a non-touristy town in that area, hop on the train to Selestat. Some of the little wine villages can be reached by public transportation.

I very much enjoyed the small city of Troyes. It's chock-full of half-timbered buildings and seems very popular with French visitors. Not many foreigners find their way there for some reason. No sign of any bus tours. My notes show that I went to the local modern art museum, but again I have forgotten the details.

We enjoyed the RS Loire Valley/South of France tour. Following is the link to my tour report, which describes the stops on the tour:

https://community.ricksteves.com/travel-forum/tours/tour-review-loire-valley-south-of-france-may-6-may-18th-2018

In the Loire Valley region, I wished we'd had time to explore Chinon. We visited the Fortress and loved it. The RS guide also describes a walk through the old town by the river. It seemed a very lovely town, surrounded by vineyards and far enough from Paris not to be overrun. It is about half an hour southwest from Azay-le-Rideau, which in turn is about half an hour southwest of Tours.

Thank you for sharing these towns. This is exactly the type that would enhance my trip. I definitely am planning to do both the Loire valley area (which I haven’t seen since 1975) and Nice with surrounding villages which will be my first time. Since I have two weeks’ time before I meet my husband to see eastern France together, those two areas will be my planned priority, plus at least two nights in Paris between those two locations.

Tourrettes-sur-Loup, Troyes & Chinon

We took the TGV from Paris to Colmar a year and a half ago. It was an easy trip and our time in Colmar was wonderful. It was a relaxing laid back rest from the pace of Paris (which we also loved) . We spent 3 days and nights in Colmar just wondering around. The canal rides were nice and the farmers market was excellent. The history of the town is fun and easy to find out about. I have heard others rave about the rest of the Alsace region. We didn't get out of town but it seemed easy enough to do. We stayed in an airbnb and had an excellent experience.
Hope you have a great time!

While in the east, don't miss Strasbourg and the northern half of the wine trail. All are delightful places. Nancy has some excellent examples of art nouveau.