Hallerbos. One of the most magical forests I've been to in Belgium.
The best time to visit this forest is from April to May. There won't be a lot of bugs and the bluebells are everywhere you look. I felt like I stepped into the filmset of Twilight. 
There are many different paths you can take good for hours and hours of walking.

The only downside of this forest is that it's too small to be called a ''forest''. I like forests that are soo big you can get lost and find beautiful secret spots you know no one or at least not that many people have been too. 
It is still big enough to walk 3 hours without walking the same route, but we've stumbled to many upon houses and the ending of this forest.
Aside from it's size the forest is gorgeous. Even without the Bluebells it has gorgeous valleys and paths. 
It's a magical place to have a walk, a run, hors ride,...
We actually wanted to have a picknick, but it's not the forest to go to if you want to have a picknick with sunny grass fields and some privacy.