Originally Posted by
Koby
Also keep in mind that you don't have to take a Thalys from Brussel to Antwerpen. There are lots of local trains doing this route. Look for "IC" trains to get the fastest (only a few minutes slower than Thalys). If you buy the ticket in Paris, they should be able to include the part on the local train.
If you choose not to buy online, do leave plenty of time to buy your train ticket--ticket lines can be brutal, and prices rise closer to departure (if you don't have a pin-and-chip CC you will need to buy your ticket at the window).