I agree with the mentioned places, however Turnhout is seen the other places have to offer to my opinion a waist of time.
The places I mention here are in Wallonia, the French speaking part of Belgium and have a distinctive character compared to those in Flanders, the Dutch speaking region. Nevertheless each place, whatever the region has it’s own charm and worth to consider.
For most here on the forum, really off the beaten path are Mons (Bergen in Flemish) and Tournai (Doornik in Flemish). The latter has an impressive cathedral, a nice lively main square with belfry and a little museum of fine arts. Mons takes about an hour from Brussels Central with the train, Tournai about 15 minutes more. There are direct trains between both places and takes a bit more than a half hour. Combining the two places will makes a daytrip more worthwile. Most exciting event in Mons is the yearly Doudou folklore festival held this year halfway June.
Even a bit faded glory, Dinant remains attractive and so touristy and offers to my opinion a bit more variety. You can visit the citadel and the home of Rudolf Sax, the inventor of the saxophone. However it takes two hours with the train to get there. The train stops in Namur too, never visited it but can be worth a short visit I think.
If you don’t have too high expectations, you can have a nice daytrip from Brussels.