I have been very impressed with Mechelen. The Grote Markt is dominated by beautiful architecture and especially by St. Rumbold's Cathedral with it's 97m tall tower, a UNESCO World Heritage Site.
There is a fee for the tower but brace yourself for the 536 steps you have to climb to get to the top. During Christmas, it was the only town that was not overcrowded. Great little market, nice food and entertainment.