Take a day trip or a couple of days and go up to Antwerp. I enjoy it more than Brussels, though I concede that I have not spent as much time in Brussels. Brugges is nice, though sort of touristy, and a bit of time in Ghent is in order if you are heading the direction of Brugges. Belgium is very (very) small, so day trips are mostly practical. Another little place that I enjoyed a lot was in Louvain, which is a bit outside of Brussels to the east. It is a smallish college town with a very nice beguinage.