There are some Thalys trains that go to Antwerp, rather than Brussels. Unless you want to go to one of the major art museums in Brussels, you might have just as much (or more) fun in Antwerp for the day. (Of course, I'd want to spend the night, rather than rush back to Paris.) I would pay for a high-speed train to make the most of a one-day trip.
Antwerp has such a variety of things to do that it is better if you do some planning. Brussels' attractions are a bit more central, so you can just walk around, although a planned walk will avoid wasting time in a city you've never been to before.
It's risky to just say so without a little current (post-Covid) checking, but many museums tend to close on Mondays.