If you were thinking of a short escapade nearby for a day or weekend, there is a two day sale (on January 23 and 24) for tickets from Brussels to Cologne. You can travel between February 4 and March 16 for EUR 15 each way in second class (EUR 25 each way in first). The trip (each way) is 2 hours and 20 minutes long, so it’s still feasible for a day trip to visit the Cathedral and do some shopping…. albeit in a bit of a rush. Tickets have to be booked online.