As expected, Thalys is having another summer sale from today through June 30. Tickets from Brussels to Paris are EUR 25 in 2nd class, EUR 55 in 1st class each way for travel between July 12 and August 31. Book ASAP because tickets are quite limited, especially on weekends.
Some tickets (EUR 15 in 2nd class, EUR 29 in 1st class – one way in both cases) to Cologne are also available for travel during the same dates.
If you can’t decide when to go this summer or plan on going later, you can always book the regular tickets at the Smilys rate (the cheapest rate at EUR 54 roundtrip) keeping in mind that you should book far ahead of time (about 2 months) to increase your chance of getting tickets.
And if you just want a quick evening trip to Paris to party in the French capital, remember they have the Thalys Night tickets for EUR 30 roundtrip, departure on Saturday night with return the following morning on the first train.