My boyfriend was planning a road trip to Hautes Fagnes (a short 2 hour drive from Antwerp, which is how long a Belgian road trip usually lasts) when he came across a cool abandoned fortress somewhere in Liège.
The fortress was originally built in 1817 (before Belgium even existed). During the two World Wars, it became a German prison. The Americans then took control of the fortress in 1945 and it was transformed into a hospital for the US Army. The fort now stands abandoned and filled with (mostly) interesting graffiti.
The complex is big, and you can spend anywhere between 1-3 hours discovering the many rooms of the fortress. Make sure to go to the rooftop, the view is great! Do try to go on a dry day, as there are lots of huge gaping holes in the ceilings and you would end up getting soaked if it's raining.
The fortress is accessible by car, there's free parking surrounding the complex. The road going up to the fortress is near Rue Thier de la Chartreuse 150, Liège. It's also easily accessible by public transport.