Belgium is a relatively small country; so flying around is not really an option to get around. There are some private charters from small local airports though and a tour over the country can also be arranged.
By Car
The most comfortable way to travel is by car. This does not only give you freedom to move without having to meet schedules. Furthermore there are a lot of sights in-between the cities that are hard to visit without your own means of transport. A dense network of highways connects all of Belgium. It’s probably the country that has the highest highway per square kilometer rate in Europe. The Belgian highways are renown for their illumination, so try to do at least some nocturnal driving.
By Train
If you don’t want to rent a car, or don’t want the hassle of finding an affordable parking place, traveling by train is very convenient. Belgium has an extensive rail net and also excursions to the neighboring countries are quite easy.
National and international passes like the Benelux Pass, which can be used to travel between Belgium, Holland and Luxembourg, are available at all major train stations.
Seat reservations are not available on national trains (except for groups over 15 persons). For some international trains passing through Belgium, reservations are available and in some cases even obligatory.
Buses connecting cities are available but not recommended.
For more info check the official railroad sites for national and international travel.
Hiking
Actually it’s not uncommon to walk or bike trough some parts of it. Especially the Southeast is well known hiking territory.
