Metz or Rouen day trip

Metz is a great place to spend a day, or three. The Museum of La Cour d'Or is one of the best museums in France and it's free. There's a great indoor market by the cathedral, the view of the Temple Neuf from Pont Moyan is prettier than anything in Strasbourg, and the old fortifications are well worth walking around if you're into that kind of thing. A little out of town is the often overlooked Museum of the 1870 war, which is extremely important in this part of France (more than WWII). Good shopping, great food, and easy to walk around, Metz is one of my favorite places.

Ideally, you should visit both Metz and Rouen. Both are interesting for the architecture and just the lay-out of the cities. I find metz fascinating, the city itself and certainly the rural surroundings, if you're deep into the history.

The museum pertaining to the War of 1870, as it is called in French, is located in the town of Gravelotte, the site of the bloodiest battle in that war.

The bus from the PEM side of the Metz Ville train station going out to Verdun also stops in Gravelotte, basically across the street from this expanded, refurbished museum. Another historical site connected to that battle is the joint Franco-German military cemetery in the town.

It's a tough choice as I really like both Metz & Rouen, and both fit your requirements.
Between Metz & Rouen has the advantage of more frequent, cheaper trains that don't really need to be booked ahead (although there are good discounts if you do).

As for Troyes and Chartres:
If you want a "medieval" vibe, Troyes is better than Rouen IMO. Less urbanized, less war damage. Troyes' churches are nothing special, but that's not a big criteria for you.
Chartres is even cheaper and faster, but churches are the main draw - there is more to explore than the cathedral and the other 2 major churches, but not that much.