The Grenspark Kalmthoutse Heide Nature Preserve is a worth-it destination run. Located 20 km north of Antwerp on the Belgian-Dutch border, the 60 km² preserve has many km of special trails around heathland, and along stately oak and beech lined avenues. The terrain also includes denser forests, grassland, a couple of lakes, and dunes. This trail map is a good overview, but even better, this web site has lots of information about the various options for well waymarked loops, including GPX files that can be downloaded. Among our favorite trails are:
Sheep Trail. 8.5 km around forest, dune, and grassland.
Castle/Gloriëtte Trail. 4.8 km. Lovely forest to a castle, then stately oak and beech avenues
Hare Trail: 4.6 km. Combination forest/heathland.
Oak Trail: 2.5 km. Lovely trail along stately oak avenues
Blueberry Trail: 2.5 km. Long avenues with American oaks