TER train with changes to Lille Flanders and from there with SNCB/NMBS IC train to Belgium and change some times to Brussel or Antwerp. 

You can also use TGV service to Lile Europe or Flanders (+10€ some seat, and after they are full 20€) and take from Lille Belgian trains. 

But this is very long. so 2-3 times more long than Thalys. 

ONLY mo-fr there are via the old=classic route that once all the INTern expresses took, again 2 connecting trains: that means from P (NOrd) to Maubeuge-Mons-BRU OR to Aulnoye-Erquelinnes/Jeumont-Charleroi, then to BRU. IN FR these trains run as TER and about ev 2 hrs- In BE its IC and ev 30 mins. Use planner  (TER-hauts de france or b-rail) for actual timings. Should be doable in about 4 hrs.

But what angelo explains- TGV to Lille for 10€, then NMBS=b-rail is for most the best ALTernative. BTW-Lille has also some fairly nice places to see, short walk from old station. AND you could make little detour via Gent-which is just as attractive but less known as Brugge. BRU itself is not overly exciting.