For me the main attraction to Düsseldorf is the beer. Five brewery taps in the Altstadt, all within a short walk of each other. And they serve their beer in .25l (about 8.5 ounces) so "one and done" is less of an issue...

If your schedule has you there on the the 3rd Tuesday of October (Oct 19), one the breweries (Uerige) releases a special beer that day

If you are into modern art, they have a couple of museums with (I'm told) good collections.

As for Wuppertal, as they say "Einmal im Leben durch Wuppertal schweben" (Once in your life you should float thru Wuppertal). I remember when I German teacher back in the day tried to describe it to me and I had no idea what she was trying to tell me