The abandoned Kinderkrankenhaus (Children's Hospital) in Weißensee, Berlin, was apparently one of the first of its kind and built during the Prussian era (before either World Wars). It’s been abandoned and left to rot. In the past year, the building has been the target of several arson attacks, but during daylight and with a bit of precaution, the site is easy to explore.
The walls are covered with graffiti both old and new, and the remnants of the children's hospital are easy to discover. Just watch where you step to avoid falling into any potholes or broken glass.