In Bruges, one of the most famous monuments is located in the city center. It is a 13th century Belfry with 48 bells. Another place to check out is De Halve Maan Brewery, which dates back to the 16th century. Today it is an independent brewery where you can take a tour, go up onto the roof for amazing views of the city, and of course sample beers! Belgium is known for many things, one of them being beer, so this brewery is definitely worthwhile and only costs six euro to get in.
In Ghent make sure to visit the medieval Gravensteen castle. It was built nine meters above ground in 1180. Today, it is open to the public to explore. There is also a museum located inside that displays many various torture mechanisms that were used centuries ago in the city of Ghent.
One of the famous sights in Brussels is called Manneken Pis (loosely translated to "Little Man Pee" in English). You will find a lot of paraphernalia with Manneken Pis on it, and the best street fries you will find are named after him. You'll be surprised by the size of the famous statue!
If you like street art, you'll want to visit the Brussels' Comic Book Route. It is a path where comic book art murals have been curated on the walls of many buildings. You'll see images from famous Belgian comics like, The Adventures of Tin Tin, Gaston, and Lucky Luke. In Brussels there are also many famous churches and buildings including the Grand Place in the center of the city, and the Gothic Hotel de Ville.
Central Station in Antwerp is a sight to see in itself, with its glass dome and marble interior. There are several beautiful churches all over the city including, St. James' Church, Cathedral of Our Lady, and Saint Boniface Church. The Royal Museum of Fine Arts is another sight in Antwerp that shouldn't be missed for those art fanatics. This museum holds works of leading Dutch masters.
If you happen to be traveling in Flanders during the winter, the Christmas markets are not to be missed. They are usually up twice a week in the months leading up to Christmas. A notable one is in Ghent; it offers ice-skating, merry go-rounds, street vendors selling Belgian craftwork, and impeccable street food. Ollibollen is a traditional holiday treat; they are small deep fried dumplings with powdered sugar on top. Be sure to try the hot wine as well!
The Flanders battlefield tour is truly an eye opening activity. The tour covers significant battlefields around the Ypres Salient and the Somme that occurred during World War One.
The 2008 film titled In Bruges has brought many people to visit the city in which it was filmed. There is a walking tour that takes you to some of the sights where certain scenes were shot.