I took an excursion from the Brussels Marriott that is brand new -- called the Hidden Treasures of Belgium. The tour guide was superb, a native of Walloon (French-speaking) Belgium, where we went. We first saw the Castle of Modave (2 photos), then with a few stops we stopped at the citadel at Namur (1 photo), then to the Cistercian abbey of Villers, my favorite, 800 years old. Since I took about 60 pictures of the latter it was hard to pick on since it is a huge complex of buildings.
I've make a reservation at my hotel's restaurant in Namur for my last dinner. Both of their restaurants look very nice and get rave reviews!