Both Thalys and Eurostar are having special Valentine’s sales these days. Eurostar is offering special deals for train tickets + hotel targeting travelers who want to travel in this season, but the deals are not that interesting (you are always much better off booking early, so you can get the cheapest EUR 80 roundtrip Brussels-London leisure fare).
Thalys’ offer is somewhat different: Tickets are on sale from today until February 20, but for travel between March 10 and April 27. The offer is as usual: EUR 25 for a one-way ticket to Paris in 2nd class (EUR 55 in first class) and EUR 15 for a one-way to Germany in 2nd class (EUR 29 in first class). This is a pretty good offer, since it is slightly cheaper than the Smilys rate and unlike it, does not require you to buy a roundtrip if you don’t need to.