If I had more skillz in Hammer, I'd definetely try making custom maps of some parts of Copenhagen.
Apart from that...
...Aperture Science Facility. There's already graffiti in there, and the charred remains of GLaDOS spurting random oneliners over the intercom would add to the feeling, methinks
Would ofc have to be some of those areas where you don't need the Portal Gun™ to get through..There are a few areas in there, though most of them are unavailable to the Portal lady