The castle is the obvious starting point — give it at least half a day, more if you book the Baroque theatre tour. But Český Krumlov rewards slower exploration too: the medieval lanes of the old town and Latrán district are worth losing yourself in, and the Vltava river loop that defines the town's shape is best understood from the water. In summer, renting a kayak and floating the circuit around the old town is one of the best things you can do here.
The historic centre is compact enough to cover on foot in a day, but the castle complex alone has multiple separate tours, and the gardens above the town deserve an unhurried visit. Arrive early or stay overnight to see the place without the midday crowds.