Greetings and salutations
You have 7 days of potential travel, starting from Paris.
1st of all, I’lld recommend you to keep to a logic, geographical route. Now you start going north, than quite far east, before slowly returning south-west. There’s no logic in that. You’ll spend too much time on trains, you won’t effectively enjoy visiting places.
I note you want to visit a christmas market, and perhaps venture through the Alps. Germany is quite well known for its Christmas markets (Weihnachtsmarkt). Basically every town or city has one in the period you’re travelling.
So I’lld start by going east, take trains from Paris Gare de l’Est towards Germany. Interesting towns and cities to stop over are Strasbourg (Fr), Colmar (Fr), Freiburg (im Breisgau) (De), Stuttgart,… It can be nice to stay in a smaller town or village in the Black Forest if you like a bit of nature. Daytime light is limited, so you might want to pick a bit of a bigger city as hide-out to have some entertainment in the evening. I wouldn’t go any further east than Munich.
Being there it’s quite easy to get to Switzerland from there (East of France, South-West Germany).
Note that reservations in Germany are optional, and non-existent in domestic Swiss trains. It can be nice to just cruise around Switzerland by train, sticking to the Alps and enjoying the views. But again, daylight is kinda limited this time of year. Other community members gladly recommend scenic Swiss train routes.
I wouldn’t go directly south, to Italy. French-Italian cross border trains are not optimal and you can lose a lot of time doing that.
From Genève in Switzerland you can get easily back to France. Nice cities you might want to discover that kinda lay on your route are Lyon, Grenoble, Avignon, Nîmes, Montpellier, Carcassonne, Toulouse… Again, I wouldn’t continue east, further than Marseille nor go back north from there.
From there you can get to Spain. Possibly via local trains at Cerbère Portbou, there’s a suboptimal high speed train connection that sells out very early for pass holders.
Spain itself I don’t know that well, but also dozens of options to visit. Other community members can come up with better suggestions than me. The climate in the south of France and Spain will be quite pleasant in comparison to temperatures North of the Alps.
For reservations, I can guide you to this link with reservation overview for all of Europe
Spain and France are considered “harder” countries for reservations. At least reservations there are more expensive than in the rest of Europe. In Spain you can only get reservations at the counters in the stations.
Basically you can do everything on my suggestions, via slower, reservation free regional trains (mainly in France and Spain). In Germany reservations are only optional.
General recommendation. Don’t rely on the rail planner app for itineraries and route scheduling, but use the national apps and planners. (You’ll be taking SNCF, Deutsche Bahn, SBB-CFF-FFS, and RENFE.) Deutsche Bahn has a good international rail planner, guess you’ll find the most information there.
