If you travel to Brussels by Eurostar, your ticket is automatically valid for onward rail travel to any Belgian city within 24 hours of arrival. Similarly, the return portion is valid back to Brussels within 24 hours of your journey home. So, for example, you could get to Brussels in the evening, check into your hotel and go for a beer.
The next morning, after a leisurely breakfast, catch a train to, say, Ghent, and have a look around. From there a boat trip along the canals will take you to Bruges, and from there you can get the train back to Brussels in the morning, giving you a day to look around before catching the evening Eurostar service back to London.