While Brussels is to be avoided for the outdoor enthousiast, right outside the city you'll find a pristine forest called the Sonian Forest. This 200 year old forest stretches out over 2000ha. You can theoretically hike here for days without seeing the same path twice, a rare thing for a forest in Flanders.
About the trail
The path is fairly easy. There are some ups and downs but all of them are rather short. You'll walk through the forest the whole time. Now and then a tunnel under the highway will remind you how close to densely populated Brussels you are.