Everywhere. First off, understand that any trip to Belgium is all about decadence. After all, the four food groups of Belgium are waffles, fries, chocolate and beer. If you like any of these things, you'll love Belgium.
Brussels is the obvious place to start. The Grand'Place in the center of town is one of Europe's best central squares: it's gorgeous. Also nice in Brussels are the large number of art nouveau bars, beautiful places to have a drink.
North of Brussels in Flanders, where Dutch (or Flemish) is the native language. This area is flat, with several beautiful old medieval cities. My favorites are Bruges and Ghent (Brugge and Gent in Flemish), both with great canals, good restaurants, and interesting markets.
Also nice in Flanders is the coast. While Ostend (Oostende) is the best known place, my favorite is a little known town called De Haan. The buildings there echo the turn of the century, making you feel like someone out of a silent movie.
South of Brussels is Wallonia, where they speak French. The land here is hillier with more woods. My favorite choice here is to take the train from Namur to Dinant. Both towns have awesome citadels worth exploring, but both are also very friendly and stroll-worthy.
Belgium is one of the true gems of Europe, with friendly people and great food. Especially the fries. And the waffles. And the beer. And the chocolate. Dang, now I've got to go back!