depends on what you want to do.
want the sea? ostend is defenetly the best city
want to go shopping? Brussels, bruges, antwerp, liege.
culture? all the big city's have a hundreds year old culture, buildings and loads of musea.
Amusement parcs? bobbejaanland, bellewaerde, walibi, plopsaland
here in Belgium you'll find all the same as in every other country.
maybe you can take a look into my hubs pabout Belgium.