If you are fortunate enough to be in Belgium during Carnival time, go to Binche. Binche carnival is overlooked due to popular celebrations in Venice and Germany, but it is SO MUCH FUN!
The Binche Carnival is three or four days of celebrations (I can’t remember). THE DAY to be there is, of course, Mardi Gras, when les Gilles (Binche-born men dressed in straw stuffed striped suits and wearing large, heavy headpieces adorned with ostrich feathers) parade the streets of Binche handing out oranges for good luck. Well, it starts as handing out and it quickly turns into throwing and pelting. It is an all-out orange war. Add to this the mass quantities of beautiful Belgian beer consumed by all, and you have a real party. You may even walk away with some lovely bruises to commemorate your experience.
Further tips- take time to visit the bars and to party with the Belgians. I lived in Belgium for a year. In general, Belgians are a bit reserved. That is not the case when they are tanked and singing along to their favorite songs at the pub during Carnival. It is just so much damn fun.
Final tip- If you are on a tight budget, stay at the hostel in Mons and bus it to Binche. When I was there in 2004, the hostel was brand new and really nice. There was a super chill bar/live music place just down the street from the hostel.