"I'll be very blunt, Brussels was one of my least favorite destinations in Europe. I think once you get past the Belgian beer, it feels like a stuffy and old city more catered to grandma and her fanny pack. Venturing out beyond Brussels was a delight."
If you were an alien from outer space and you only had a few days to get a very broad idea of Europe, you can't go wrong with Brussels. The buildings are old, the architecture very much European and that's pretty much it. I think when in this area, you can do yourself a great favor in venturing out. The city has great connections to a wide net of Belgium. During my time there I went to meet a friend in Antwerp and had an absolute blast. I feel that this is probably the case in Amsterdam. Once you get over the red-light district and weed stuff there isn't much else, but venturing out to its lesser known towns it feels refreshing.
It's a bit out of the way to get to, but the Atomium is a worth-visit I think. It was built for the 1958 World's Fair. On a sunny day its super shiny and impressive. Inside, there are traveling exhibitions.