>>2862735I walked a bit through the Ardennes a few years ago, mostly from Dinant to Charleville in France.
Big forrest, the Meuse valley, some lakes too, you wil leasily find war museums and maybe abandoned forts and factories to explore.
I visited the Groningen region, it's clearly not suited for wild camping, so much canals you can't access most of the fields just next to the the roads. Otherwise peaceful and flat, if you like swamps and birdwatching this is a great place. For biking too obviously.
Free public transportation in Luxembourg may be useful to travel in the area.