Thank you for your response.
I will be staying in Düsseldorf during my December trip, from where it is only a short hop to Venlo.
I think maybe I will take the bus to Maastricht. I was there at the end of November last year and sadly just a couple of days too early for its Christmas market; maybe this year I can actually catch it in full flow.
The journey on public transport from Düsseldorf to Valkenburg would be too time consuming.
If you are staying in Düsseldorf I would just visit the Christmas market there. Christmas markets originated in Germany and the Christmas market in Düsseldorf is one of the largest and most famous ones.
Going to Maastricht from Düsseldorf to see a Christmas market is a bit like flying from Paris to Las Vegas to see the replica of the Eiffel Tower. If you do insist on seeing a Dutch Christmas market I would still suggest on going the Valkenburg. The Christmas market in Valkenburg is the oldest in the Netherland plus it’s held in caved which makes it different from the Christmas market in Maastricht.
Thank you once again for your input. Much appreciated.
I will look again at perhaps finding a way to Valkenburg from Düsseldorf for a day trip.
I have been to the Cologne Christmas market several times over the years including during the 2017 season, so I will probably skip it this year, though thank you for the suggestion.
This year I am travelling from Berlin (7 nights) to Düsseldorf (5 nights), stopping in Halle an der Saale (5 nights) en route. During my stay in Düsseldorf I hope to visit Münster and the Netherlands as well as probably one other place. Bonn, perhaps. Not Aachen as I was there too last year.
I still have plenty of time to finalise the itinerary!