Day trip from Paris to Brussels

I think you will definitely want to use a high speed train to get to and from Brussels in order to maximize your time there. Look at the official French train website https://www.sncf-connect.com/en-en

Perhaps someone will reply with a specific guide to recommend, but if not I would probably try contacting the Brussels tourist office, or a website such as Get Your Guide for ideas. A group day tour might also be an option if they cover the sights that interest you.

Best way is the Eurostar highspeed train from Paris-Nord to Brussels-Midi. (eventually followed by a local 5 minutes train ride to Brussels-Central station in the city center).

Private guided tour https://guidesbrussels.be/home/