In April I am going to Netherlands for a week with my son and daughter-in-law. I would like to add a couple of days to visit Germany, Belgium, or ????? Any recommendations?
Any part of Germany can be reached within one day's train ride, either shorter or longer. See https://www.ricksteves.com/europe/germany, for a start. How to Look Up Train Schedules and Routes Online gives you the DB train schedule link and tips for using it.
You might re-post this query on the Germany forum so the hard-core Germany experts can weigh in with their advice.
A day in Bruges and a day in Brussels is nice. You could also have a quick stop at Antwerp as you make your way to/from Netherlands. I hope you are planning to see Den Hague, Rotterdam, and Kinderdijk on your trip in Netherlands. It is a great country.
Thalys train from Amsterdam Centraal to Paris takes 3 hours. Book directly on the Thalys web site and you can get great deals.
Museums , churches, Eiffel Tower, croissants, crepes, Croque- Monsieur.
Going overnight in December; wish I had more time but I'll take what I can.