Just returned from three nights in Brussels. Currently the city is running a Festival of Lights, with special lighting events taking place all over and a route between the two dozen major installations. I couldn't get tickets to the show inside the main cathedral which was a huge disappointment. They sell out. Also, between 5pm and 8pm there will be thousands of people out walking the route with friends and family, Quite impressive since the temps have been in the 20's. Rain and/or snow daily. The light show on the Palace Royal is fun, and the palm trees in the park is, I think, something they should leave up all year.
The Museum of the Army is excellent, although the wing with the aircraft is partially closed due to construction. I'd love to see them clean those up and actually get some restored to flying condition, they have some very rare pieces.
Autoworld is really good too, although the current "temporary" exhibit is about MBZ, most of which are cars from Stuttgart.
People start lining up for entry in the Beau-Arts more than 30 minutes early; by opening time it's blocks long. Make sure you get your ticket ahead of time.
The Museum of Natural History is THE place for families with kids. It was jammed on Sunday afternoon. Very popular! And not just the Dinos, but every floor had crowds. It's really well laid out, and most of the displays are in English, Flemish, French, and German. The mineral displays are not in English, but quite impressive.
The Sablon Antiques market is okay, but frankly the dozens of other shops and galleries in this part of the city have better stuff. The problem is you need to work around some of the opening times, which are not standard. There's also a dozen or so tents behind the Notre Dame during the daytime.
The Galeries Royales Saint Hubert is an exceptional piece of architecture, and very popular with the tour groups, but expensive. If you want chocolate look for the shops away from here or the square around the town hall. Don't be afraid to ask for a sample before you buy.
The Palace of Justice is completely covered in scaffolding. The Ferris Wheel which is right there is good for a view at night.
Remember, this is a town with hills, and really bad sidewalks. Loose tiles, missing stones, and very uneven surfaces make getting around more of a challenge than you might expect. Also be aware that parking sucks, and is expensive. But public transportation is very good, and you can just use your credit card to tap so you don't need to buy a pass card, which I find much better than Paris, although not as good as Luxembourg (where it's all free).
Coming from Germany I'm also very surprised at how many homeless there were, and the large amount of trash on the streets. (It's not just in the city, even the roads have a lot of trash on the sides).
Stopped by the Waterloo battlefield on the way home. Good, but small, museum. The Panorama is very good. But the Lion Monument is totally covered in scaffolding, which was very disappointing. If you go, and it's been raining, be advised that the area off the paths is very slippery.