pizzo: I don't disagree, but it does help to have a starting point as to what they are already looking at and/or their interests. If a poster knows nothing about a destination, even an out of date guidebook will be much more helpful than this forum.

Perhaps the responses would have been different if the OP had said something like >>My partner has a conference in Brussels and very last minute I've decided to join him. I know nothing about Brussels/Belgium. Where should I start?<<

Now MarkBubbling has returned and posted essentially that. It doesn't sound like he was terribly offended. Just got off on the wrong foot a bit. So no need to slam each other for what/how things were said.

You should visit www.visitbrussels.be first.

The official TI website has up-to-date infos on museums, tours, opening hours etc. No need to spend money on a guide book (though I must admit I like to travel with one).

Aside from the well-known museums like the Magritte, you may also find (usually temporary) exhibitions at the cultural center Bozar. www.bozar.be
The Cartoon Museum has quite a reputation, but IMO it is a bit more for the true BD aficionado than for the regular tourist.
The Atomium looks quite impressive, but also has a quite impressive entrance fee.
While Brussels' function as the EU's capital is more or less limited to a somewhat random mix of questionable architecture between Place Luxembourg and Schuman, you can visit the Parlamentarium at Place Luxembourg for free (maybe only as a backup activity for mondays -- only PM hours then, but when many other museums are closed all day)

The fine examples of Art Nouveau buildings and interiors have already been mentioned. Google "Victor Horta" for some landmark examples you can visit in Brussels.

When it comes to dining out, or just having a few drinks, I must admit that I am biased against the Grand Place and surrounding streets.
The are many neighborhoods outside the inner ring road which I find more interesting and less pricey than the town center.

A good place to look for restaurants, pubs, or cafes can be around Rue Boniface @ Rue de la Paix, or at and around Place Flagey - both in Ixelles/Elsene.
In summer, also Place Sainte-Catherine has a nice flair, IMO.

If you only have one day for a day trip, you should go to Brugge. Or Gent. Or Antwerpen.

If you feel crazy, you can go to Paris for a day trip.
The Thalys highspeed trains take you there in 82 minutes.
While one may question if just one day is "enough", it can be somewhat tempting