Brussels for a day

The central train station in Brussels is in the middle of town, you can walk the city from there. You won't need metro connections except perhaps in Paris. You may want to take a bus up to the main museums, it's a good walk.

What kinds of things are you interested in? Just walking around the squares, visiting a couple of museums, be sure to find some chocolate shops (they will find you, actually, just follow your nose).

The Thalys train from Paris ends at Brussels Midi/Zuid station. But you can just hop another train for free to get to Brussels Central/Centraal station. From that station, most sights are within walking distance. Of course you want to see Grand Place and the wonderful shopping arcade, Galeries Royales St. Hubert. I really like both the Magritte Museum and the MIM--Musical Instruments Museum. The food is great here, so be sure to give yourself some time to have a leisurely lunch. And there is always chocolate...

If you want to do this, be sure to book your tickets as soon as you know your travel date. That route is run by Thalys, and their prices start out low for non-exchangeable, non-refundable tickets (a quick look shows €29 each way). They go WAY up as you get closer to the travel date or if you need more flexibility (trains for tomorrow are €99 each way!)

The website is https://www.thalys.com/be/en/, and it seems they sell tickets up to 3 months ahead.

Spend at least one night in Brussels

As for what to see, I'm not a Brussels fan, so I'll leave that to others. But I loved my Art Nouveau tour with ARAU. If I ever go back, I'll plan my visit around the dates of their English language tours: http://www.arau.org/en/tours

Aarau tours are fantastic.
Go for an evening at the opera de la Monnaie
Visit Museedu Cinquantenaire, or Central Africa in Tervuren.
Do talk to people in Brussels. People are very friendly and like to share experience.

Florence - The Museum of Central Africa is alas closed for renovation, plan is to reopen in 2017. Parts of the collection are on display in several pop-up exhibitions elsewhere. Somebody on Lonely Planet described till just before closure as it was a museum in a museum, and I see it the same way. It was like a time capsule how people looked still through the “colonial” eyes to the world not so long ago, having now an update.

For going to Musée du Cinquantenaire/Jubelparkmuseum (Royal museums for Art and History) you can take the metro from Brussels Central station.

I can reccomend to visit any flea market! I visited a lot of them and I think in Brussels there are the best ones. I love the atmosphere and you also buy smth

what's smth? That's a new one on me.

Probably "something".