If any city can claim the title "Venice of the North" then it has to be Bruges. In the 13th century it was one of the most important cities in the world and its medieval glory has been almost fully preserved. Its museums and churches are ornamented with works by Hieronymus Bosch and Michelangelo.
Beautiful old town where you must rent a bike and ride the back alleys along the water. Has that untouched feeling that is so special and rare... but of course you must not go in the Summer. I was there in December and maybe saw four tourists.