Český Krumlov, in Bohemia’s deep south, is one of the most picturesque towns in Europe. It’s a little like Prague in miniature – a Unesco World Heritage Site with a stunning castle above the Vltava River, an old town square, Renaissance and baroque architecture, and hordes of tourists milling through the streets – but all on a smaller scale; you can walk from one side of town to the other in 20 minutes. There are plenty of lively bars and riverside picnic spots – in summer it’s a popular hang-out for backpackers. It can be a magical place in winter, though, when the crowds are gone and the castle is blanketed in snow.
Český Krumlov is best approached as an overnight destination; it's too far for a comfortable day trip from Prague. Consider staying at least two nights, and spend one of the days hiking or biking in the surrounding woods and fields.