A quick getaway from Maastricht to the south across the Belgium border, about half an hour from Liege, are the Les Grottes de Remouchamps, hidden stalactites underground caves and underground rivers with guided tours and boat rides taking you through the complex underground system. There’s typically no English available, but for some reason when I was visiting there were lots of tourists and so the guide did a tri-lingual Flemish/German/English tour which worked out pretty well. These aren’t the nicest underground caves I’ve visited, but it has the added twist of the boat tour and river that you don’t find that often in such caves…
Location:
A fun way to spend an hour or two, not too far is Spa for some thermal hotspring in a historical Roman spa town.