There's some regions where you can get some nice riding done. But it takes a bit of effort to stay legal. Forrests are totally off limits to motorized vehicles in Flanders.
The last few years have seen more strict rules applied in the south as well. Genereally forests are off-limits there as well, unless they are privately owned.

Nevertheless, still some nice riding to be had outside the woodland areas:
http://www.advrider.com/forums/showthread.php?t=537190
http://www.advrider.com/forums/showthread.php?t=736341