I don’t know about the accommodation, but I do know about the area, as I have a holiday house nearby.
It really is underrated.
You will not be short of activities, as there is plenty of scope for cycling (both road and mountain), as well as kayaking and climbing.
If you like beer, the various monasteries that still make their own are easy to access, including Rochefort – which is a beautiful little town in itself.
If you like books, there is a village called Redu that has sought to become the Hay-on-Wye of Belgium. It’s full of used and antiquarian bookshops. Just make sure you take cash with you, and don’t depend on your bank card.
As far as cycling goes, there is a network of mountain bike trails (for example, http://www.ardennes.com/accueil/a-voir-a-faire/les-activites/a-vtt-dans-les-ardennes) throughout the region, as well as a road cycling network. (I assume you don’t need me to tell you that, when searching for routes and/or resources, use the term “VTT” for “mountain bike”.) The road cycling network is called RAVel.