Christmas Markets / Stuttgart / Esslingen & Ludwigsburg in One day?

Slow down. Most of the better markets are best savored in the evenings after the locals get off work. They're about a block party, not about buying stuff.

I've been to both a few times. Is it possible, sure. It's only about 30-35 minutes by train from one to the other on the S-Bahn with a change of trains at the Haupbahnhof (same platform so no long connection). the markets don't open until 11:00, so no need to get an early start. I would start with Ludwigsburg and plan a few hours. It's fairly compact in the main square so you can't get lost. If you're interested, the Ludwigsburg Palace is only a short walk from the market (https://www.schloss-ludwigsburg.de/en/home).

I particularly like the Esslingen market in the evening. The market is in the middle of town surrounded by timber framed buildings. The evening lights add to the charm. Gulwein is the drink of choice at most markets including Esslingen, but I would try the warm Honey Mead served at one of the stall in the Medieval part.

Since you are staying in Stuttgart, don't overlook the market in the city. It's one of the largest in Germany with booths throughout the center of town. You can't miss the market if you head down KonigStrasse.
Enjoy!

reupsgt, thanks for the suggestions, I think that will be the plan.
We will be doing the Stuttgart markets the day before.
Not sure we will have time for another town, so Heidleberg will be saved for the next time!
On our last half-day morning, the plan is for the Mercedes museum. Do you all agree that's worth the visit?
Thanks for the input/
Safe travels to all.

The Mercedes Benz museum is really great, it even convinced my car-hating eco-warrior partner. Highly recommended.
The Porsche museum, you can give a miss unless you are a real petrolhead.

Pat, honestly I haven't been to the Mercedes museum but I've been told it's worth the visit.

I agree with balso - the Mercedes museum is really excellent, but the Porsche one is only interesting if you are a big Porsche fan - too many variations on the 911.