Hackesche Höfe is a linked set of eight courtyards near Hackescher Markt, restored into one of Mitte's best-known courtyard complexes. The draw is the mix of Jugendstil detail, narrow passages, shops, galleries, and cafes packed into a dense piece of old Berlin.
It works best as a short wander rather than a single monument: step through the courtyards, look up at the tiled facades, and use it as a starting point for exploring the streets around the Spandauer Vorstadt.