Thalys has a special sale: EUR 25 each way to Paris in 2nd class (EUR 50 in 1st) and EUR 19 one way to Aachen or Cologne (EUR 29 in 1st).
The sale started yesterday (Nov 30) and runs through December 20, and is good for travel between December 15 and March 16, 2008. If you’re planning to travel on weekends, book early as seats are limited. Also remember that you can only book Thalys tickets up to 3 months in advance, so if you want to travel on the last day of the offer, you will have to wait until mid December to be able to buy your ticket.
Offer is only bookable online.