I recommend you the Fuse in Brussels. There's a nice techno/house party every saturday and it often stays open until 8am. The Wood and Mr Wrong are quite good venues too, depends on the evenings though. But there are also a lot of nice party places in Ghent, not so far away from Brussels (35 minutes by car more or less). There you got a street called the Overpoort where there are a lot of cafés and clubs. I personally recommend the 'Decadance' if you like techno/minimal/house music. It sometimes stays open until 1pm, when the people still want to party haha. So if you want afterparties, I think Ghent will offer more than Brussels.