Grand Place (literally)
Brussels is a beautiful city, with a great deal of things to see. We spent 2 days there, but I think one more would have been ideal to enter the museums and attractions. It is a fair sized city but is totally possible to go on walking to most places, although the metro system is very good and easy to use.
Insider Tip:Museum Magritte is very nice and you can buy a combo to other two museums near it for a cheap price. Try to do the Comic Strips' walk, there are several murals around the city depicting famous characters like Tintin and Asterix.
If you will be there for only two days, it is best to buy only a 24h ticket to the metro, because it actually entitles you to the day you buy and the next one.