(2-part post):
I love Belgium. It's really easy to get around by train.
If you're arriving by Eurostar, remember that your ticket to Brussels includes free onward travel to any rail station in Belgium. (You can also travel free to Brussels Midi, from any rail station in Belgium, on the date of your return travel).
If you're flying with Ryanair, you'll land at Charleroi. You can buy a 'bulk ticket' at Charleroi ticket office for �10.50, which will take you to any rail station in Belgium. (It includes the bus to Charleroi Sud rail station). You can buy your return ticket at the same time. (It won't matter if you'll be returning from a different point. You simply fill in the station you're travelling to, or from, together with the day & date of travel on the ticket).
Another bargain is the Belgian Rail pass. You get 10 single journeys, anywhere in Belgium, for just �69. (That's about �5 per journey). Just make sure that you understand how to fill in the pass! See here for details:
http://www.b-rail.be/nat/E/tarifs/passes/rail/ index.php
I suggest basing yourself, for a few days at least, in Brugge (which is a beautiful city).
http://www.brugge.be/internet/en/index.htm