The Demer river valley between Aarschot and Diest is extremely well served by the Flemish network of footpaths. The area includes a number of nature reserves, and is generally a lovely, peaceful and scenic area for a short Saturday or Sunday afternoon stroll.
A good starting point is Testelt railway station, which is on the Leuven-Hasselt line, and where there’s plenty of parking if you’re arriving by car rather than train.
Starting from junction 92 by the station, a pleasant circular 9.5 km walk follows junctions 143, 175, 298, 60, 246, 374, 208, 134, 59, 396, 145 and back to 92. Here is the route to download from RouteYou. And here are some photos of the walk.