Bernau's medieval town walls and gates are the main draw — the Steintor, Werdertor, and Mühlentor survive in unusually good condition and are best seen on a short walking circuit of the old town. The Heimatmuseum inside the Steintor covers 500 years of local history and is worth an hour.
For architecture enthusiasts, the real prize is the ADGB Trade Union School about 4km north: a 1928 Bauhaus masterpiece by Hannes Meyer, now a UNESCO World Heritage Site. Combine it with a walk into Barnim Nature Park for a full day out of Berlin. The Pulverturm — a converted medieval gunpowder tower — hosts the Wolf Kahlen Intermedia Arts Museum, an eccentric and worthwhile stop for anyone interested in media art.