Well... if you're going to do that whole route, I would suggest perhaps a night or 2 in
Dusseldorf. You might want to sandwich it between Amsterdam and Cologne since it's between the two, right on the way to Cologne. I never got to go but always wanted to. I spent a very short time in Frankfurt which was ok but I don't recommend... very different from the rest of Germany... very modern... looks like any big city in the US. Airport is freaking HUGE though haha.
I was originally considering Amsterdam and Belgium also when I went last summer and opted for more time in Germany and 4 days in Poland. But I did ask around and as far as Belgium goes, everyone I've talked to has suggested Bruges over Brussels.
Another Germany suggestion is
Bremen, which is kind of on the way right after Amsterdam. Another place for right after Cologne but before Stuttgart is
Mannheim. We passed through there on the train and I always wished I would have gotten off there and wandered around a bit. It's another city on the Rhine.
Also, try not to plan Heidelberg for a Sunday... I went on a Sunday and a lot of the quaint little shops and stuff were closed for the day!
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