Be careful not to try to pack too much in. Amsterdam to Munich is a long way, and if you try to fit three other places in en-route, you are likely to spend all your time travelling, and will be too rushed to appreciate anything.

This is the number one mistake people make on multi-centre trips.

For an itinerary this short, I would suggest sticking with the places close by to Amsterdam. This offers the options of:

Aachen: a locely city next to the Belgian and Dutch borders, with a stunning cathedral.

Köln (Cologne): a pretty city on the Rhein, famous for its Gothic Architecture.

Trier: a lovely quiet town in the Mosel winelands.

Koblenz: an ancient city at the junction of the Rhein and Mosel rivers.

I like the idea of a river cruise. This would provide a good taste of the region with minimal planning or hard work.