Don’t make the mistake of just seeing one part of Brussels. There are two "towns" separated by elevation. The Lower Town is just that, the part of the city at the bottom of the big hill. There, you’ll find the Grand Place, the Manneken Pis, the chocolate shops, etc.
If you want to walk off all the chocolate and waffles, take a 15 minute walk UP the hill to get to the Upper Town (or, spend 1.50 Euro to ride the bus, #95, which you catch on the side of the Bourse, or Stock Exchange, just off the Grand Place.)
There are lots of sights that await you - including one heck of a view back down on Lower Town. The bus drops you at the Place Royal (notice, it’s not PALACE, the palace is around the corner.) The statue here stares down at the Town Hall, and you should, too. The view down the street is one heck of a picture.
Once you’ve crossed back across the large street dodging trams, buses of tourists and cars like the frog in Frogger, take a peak around the corner. There you’ll find the Palais Royale, or Royal Palace. The gardens in front are really cool, and I’m sure if we’d walked through the paths of well-cut shrubbery, we would’ve found ourselves in the middle of a maze. The palace itself makes for a great photo op, as does the park across the street, Brussels Park.
The rest of the walk is detailed in Rick Steves’ book, Amsterdam, Bruges and Brussels so I won’t plagiarize him. However, there is plenty to see in Upper Town, and the point is that you should make the trek.
I will say that if you are limited in time, say you have a layover or you’re just stopping over on a train trip somewhere else, I’d concentrate my time on the Lower Town. That said, if you can, Upper Town doesn’t disappoint.