Only until this Friday!
Thalys has a special offer this week for tickets to Paris for just 22 euro each way. You only have four days to book your tickets (last day is June 4, 2010), but you can travel anytime between July 5 and August 27 (provided you find tickets at that special rate for the dates you wish to travel…)
As always with these offers, the earlier you book, the better your chances of finding tickets for the dates you’re interested in.
If you prefer to go to Germany (Aachen, Cologne) instead of Paris, it’s just 15 euro each way. Book online and see more details here.