Thalys is having another online sale: a two-day sale starting on Wednesday, July 18. Tickets from Brussels to Paris are EUR 19 one way in 2nd class (or EUR 29 in 1st class). Travel is allowed from August 1st through September 7th. Book early, especially if you want to travel on weekends.
Note: Thalys has online sales throughout the year, but the most interesting offers tend to appear in the summer time, when business travel declines and they still need to fill up their trains.