Thanks for an answer. I went once to Belgium, just before my trip to Pyrenees last year, to train a little bit. I was in the region between Namur, Liege and Leuven. However I found those roads a bit dull.
I mentioned south of Belgium, because i have found already on some other forums that people recommend that region. Second ago I bought a Michelin map from Amazon and will just ride there for the weekend :-). I thought, that maybe someone knows some particularly nice road or region down there.