Distances are short, and the public transport is good. You can of course drive around for total freedom, but with such good public transport, alternatives might a good idea for both your wallet and the environment.
The Belgian railway system is run by the SNCB/NMBS and is the most dense in the world (see the area maps). Trains run frequently, at least once every hour and up to six times an hour in peak periods.
The Airport Express connects Brussels Airport with the city 4 times an hour between 5.24 and 23.46.
Trains run to every possible place in the country: from Brussels to Antwerp (5 trains/hour), Ghent and Brugge (3 trains/hour), Namur (3 trains/hour) and Charleroi (2 trains/hour), amongst many others.
Young people (-26) can use the Go-Pass (10 rides for 41.50 euro). Senior citizens (+65) get a return ticket to any destination within Belgium for just 3 euro. Everybody else can get a railpass: 10 rides for 62 euro.
International trains are available, and are popular for short trips to nearby cities in neighbouring countries. Check the site of the SNCB/NMBS for more information on the regular international train service. Noteworthy are the high-speed trains Eurostar from Brussels to London, the Thalys from Brussels to Amsterdam, Paris and Köln and the TGV to several destinations in France like Paris, Lille, Bordeau and Marseille. They are ecologic alternatives to flying and, ironicaly, also faster.
The SNCB/NMBS webpage offers the posiblity to buy national and international tickets online.
The bus network is even more dense than the railway system. It is run by three companies, one for every Belgian region. Buses are excellent for city transport, or transport over short distances. In Brussels, public transport is handled by the MIVB/STIB, in Flanders by De Lijn, and in Wallonia by TEC. Each company offers schedules and route planners on their websites. Bus fares are relatively cheap. One-use tickets are between 1 and 1.5 euro's. 10-use tickets are cheaper.
The main Belgian airport is Brussels Airport. It is connected with the downtown by the Airport Express Train (see RAIL section)
After 2001, Brussels airport recovered from the bankruptcy of Belgium's renowed national carrier Sabena. Sabena was replaced by SN Brussels Airlines, which merged with Richard Branson's Virgin Express into Brussels Airlines. BA now serves an extensive list of mainly European, African and North-American destinations. They have some good offers on European journeys as they try to compete with low-cost companies. Low-cost career Vueling has really cheap offers between Brussels and Madrid/Barcelona/Valencia.
Trans-atlantic journeys are often cheaper from/to Brussels than other European capitals. Check out Joker for cheap trans-atlantic flights from/to Brussels.
Ryanair has been using Charleroi Brussels-South airport as its main hubs on the European continent. Other low-cost carriers, like Wizz, also use the airport now. It is located south of Brussels, about 1 hour on the train away from the capital city. Frquent bus-connections link Brussels' Midi-station with the airport.
As of mid-2004, destinations from Charleroi include Budapest (Hungary), Carcassonne (France), Dublin and Shannon (Ireland), Girona and Valladolid (Spain), Glasgow (UK), Warschau, Katowice, Poznan and Krakow (Poland), Rome, Venezia, Milan, Pisa and Pescara (Italy), and finally Stockholm (Sweden).
Antwerpen-Deurne airport is served by three low-cost airlines. VLM is the Flemish airline carrier and uses Antwerp as its main hub with connections to London, Jersy, Liverpool and Manchester (all UK). Discovery Link flies to Prague (Czech Republic). Welcome Air flies to Innsbruck (Austria).
Along canals, old railroads and quiet countryside roads you can find well-indicated long-distance routes (yellow-blue marked or metal plates).
The northern part of Belgium, Flanders, is quite flat. The routes are numbered from LF1 to LF50 (not all finished). For example the LF1 mainly follows the North Sea, from Northern Holland to France.
The southern part, Wallonia, is hilly. Three routes are actually marked: RV2, RV7 and RV60. Besides a network of flat roads is under construction ('Ravel'), on old railroads and along canals. Actually finished: Ravel 1, 2, 3 (from Brussels to Erquelinnes at the french border)
Hitchhiking is pretty easy. Along main roads and at the entrances and gasstations of highways you probably won't stand very long before getting a lift. Remember there are different languages in different regions. English will get you a long way in Flanders, less in Wallonia. Most Flemish people understand and speak French, but most young people prefer English. Your knowledge of Dutch is not worth much in most parts of Wallonia. Places can have different names in the two regions (e.g. Brussel/Bruxelles, Antwerpen/Anvers, Luik/Liège, Gent/Gand, Brugge/Bruges,...) and they do not always resemble. To hitchhike to France try the highway to Mons/Bergen.
For national or international carsharing check out Taxistop