I will be in Brussels the second week of June. I am coming from a week in Paris. I assume the same attire is appropriate in Brussels as in Paris. I also assume I should expect rain. As I come from a cold climate, I will be wearing short-sleeved shirts, but bringing a jacket. Nice pants and skirts in neutral colors; mix and match and quick drying. Plus comfortable walking shoes.
I know this is an unpopular question sometimes, especially with the gentleman. So if you are not interested in clothes and fashion, you don't need to bother. :D Meant in the nicest possible way.