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!