Agree with
Stadjer , once out of Brussels, it's safe. There are safe cycling routes everywhere, just depends on where you want to go. I've since been to The Hague, which is nice, as are Kinderdijk and Gouda. If you have time, consider Bruges. I've never been, but hear it's nice. The website I used for planning my route is finicky, and I'm sure the app recommended by
djb is much better.