Search "Ardennes" on here and you'll find loads of info.
There are some lovely roads, anywhere South of Dinant, basically. Unlike this country where the occasional decent roads is notworthy are identifiable, pretty much anywhere in the mountain area will be lovely.
My favourite little area is to the East of Montherme, Along the Semois river, in the little bit of France that sticks up into the Ardennes.
Anywhere round Spa Francorchampos will be nice, and there might be something worth watching at the circuit.
The highest point in Belgium is Haute Fagne, which has a very fast road up to it!
Luxembourg is a funny old place, but also full of lovely roads away from Luxembourg City. Diekirch is a nice town to stay in.
Finally, the key word in NeilDD's post is "waterproof."