The best way to travel trough Belgium (and surrounding countries) is undoubtedly by train. Brussels has great railway connections, as it not only has several train stations spread around the city, as it has trains leaving to everywhere in every hour. Try to do the exercise of going to Gare Central, for example, and checking the departures' screens. You will see three screens full of information on trains leaving to all over Belgium and even abroad in the period of just 1h. So it is really convenient, cheap and fast to travel by train here.
As for prices, the best option for trainees and young people in general (under 26 years old) is to buy the Go Pass 10, which is valid for one year and allows you to travel ten times from and to anywhere in Belgium for just 50€ (so each trip will have the value of 5€). If you're over 26, you have the same kind of ticket, but it is called Rail Pass and it costs 76€ for 10 trips as well (so each trip will have the value of 7,60€). Both these passes can be used by more than one person, so if you're traveling with a group of friends you can buy only one pass and just fill it with two journeys. It is also great that you can enter the train directly and don't have to lose time on queues to buy tickets each time you're traveling (which could be a hassle, as there are many tourists in Belgium and usually the queues are big).
The Belgian rail website is available in English and is quite informative. You can simulate the journey you want to make and they show you the schedules, train changes and even the platform from where your train leaves and where it will arrive. So it's really easy to plan your journeys and do some daytrips from Brussels on weekends and holidays.