Brussels had its own dialect for a while. A mix of mainly Flemish dialect, with a bit of French and even a dash of Hebrew and Spanish, this dialect is now rarely spoken, but a lot of words still have sprung up with the French-speakers in Brussels. So here is a little vocabulary.
First of all, Brussels is a city of Zinneke: a mutt, mixed-blood dog, and by extension now, a multi-cultural person. Brussels is so proud of being a Zinneke that it has the huge Zinneke Parade once every two years. Usually, four different processions start at the different geographic corners and join in the center of Brussels, celebrating the wealth of the melting-pot Brussels is.
Brussels is a city that where the "Zwanze" is alive! Zwanze is a joke, and by extension, describes a predilection to play tricks and have a good time. If you're that kind of person, then you are likely a "zwanzer".
If you are a young boy, don't be surprised if you're being called a "ketje" (or ket) -- that's an affectionate little word for a boy or a young man. Older men can be referred to as "peï".
Also, if you end up at the "Amigo", it could be one of Brussels's most luxurious hotels, but it is also the name of the city jail.
For French-speakers, see the link for an interesting site about "Brusselers": http://www.eurobru.com/visit16.htm