Cesky Krumlov, or Krumau in German, is a gem of a village, but not really a secret anymore. Lots of people have discovered it, and it would be advisable to visit outside of high season. The essence of the town is medieval, but there are Renaissance and Baroque monuments as well, and the Vltava River meandering through town gives it an even nicer atmosphere. Because it used to be way off the beaten track, it usually escaped marauding armies and was thus able to preserve its historic townscape. It is probably too far from either Prague or Vienna for a daytrip (but certainly doable if you want to), but there's enough to see there for two or three days anyway.