Not on a plane but you would easily find weed in Hamburg and don't carry anything to Copenhagen. That's tricky though because what you find in Amsterdam you would want to keep on consuming for a while after leaving Amsterdam
Never been to Hamburg, but of ze kraut cities, Berlin is defenitely one I would recommend the most. And you would defenitely not even bother to look for weed there (or most of these cities), they are just around in the typical spots (prolly Prague was the easiest for some reason).