Paris to Berlin

It’s difficult to suggest without knowing what your interests are, or whether you mean a few hours or a few days. 

If you take the southern route, then Strasbourg is an excellent place to stop, and well worth a visit - although it’s only a couple of hours from Paris.

You can also take the northern route, through Belgium, you could stop in Lille, Brussels, Cologne. You also have a sleeper option from Paris or Brussels to Berlin.

Maybe Frankfurt its on the way and there is plenty of stuff to do there on a short stop inbetween. 

Brussels is a fantastic city to visit if you decide to take the route through Belgium from Paris to Berlin.