I visited Brussels for a couple of days and found it very interesting!
You must visit the Grand Place! It is beautiful, especially at night, when it's all lit up. There are a lot of cool buildings there, such as the Town Hall. It's also good to go at night because... the streets that surround it are full of restaurants, so you'll definitely find somewhere to eat. However, try and ignore too persistent and pesky restaurant managers who try to lure you into their restaurants, make your own decision over where you want to dine.
If you're looking for discos, just be careful where you get yourself into.
I almost accidentally entered a gay disco thinking it was a normal one haha. I don't really have anything against gay people, but I'm just saying in case you do.
All in all, I found Brussels really interesting and fun, but found Brugge a lot better, so if you're visiting Belgium, make sure to go to Brugge.
If you have any tips or comments of your own, please feel free to share them!