Do not miss Luxembourg if you are in the Ardennes, or at Bastogne, Belgium, for Battle of the Bulge matters related to WWI and WWII. It is close.
You need not do more than an overnight, if you are pressed for time and get an early start for the first day. Pay some tribute to General Patton, buried here at Hamm Military Cemetery outside Luxembourg City. See worldatwar.net/nations/luxembourg/narrative. See castles, then spend an overnight in Luxembourg City at least.
In Luxembourg City, there are the city walls with the old town inside.
More blogs about Luxembourg Road Ways.