Rick Steves' walks through the lower and upper town are nice. If you only have time for one I would do the lower town walk.
I enjoyed looking at the Manneken Pis costumes in the city museum.
If you are at all interested in architecture, walk to the upper town to look at the Old English building - it a wonderful Art Nouveau building. The museum inside (musical instruments) is okay, but the building is definitely worth the walk.
If you are interested in comics, you can get a brochure from the tourist office or comic book museum (skip this as it is all in French or Dutch, however it is inside another beautiful Art Nouveau building) outling a comic walk through town. There are some 30+ murals and statues through out the city of comic book characters. (You can also find it online at http://www.brusselscomics.com/en/route_bd.cfm)
There are a couple art museums in town, but I don't remember any of them being particularly memorable.
And don't forget to try some of the wonderful chocolate located around the square: Godiva, Gallers, Leonidas, Nuehaus...
Delirium Cafe/Bar is a wonderful place to spend the evening -they have around 2000 beers on the menu. It is a great place to try something new; we had a lovely banana beer and an entire range of Christmas beers it would have been next to impossible to get in the states.