The hook
A medieval town wrapped inside a tight double bend of the Vltava, under a castle with a painted tower — and a river the Czechs treat as a summer highway, so the best view of the place is from a rented raft drifting through the middle of it.
01Why it's worth it
Krumlov's shape is the appeal. The river loops back on itself so sharply that the old town sits on what is nearly an island, which means almost every street ends at water and the whole thing is walkable in an afternoon. Above it the castle sprawls along a ridge — the second-largest in the country — with a round tower painted in trompe-l'œil and, in the moat, bears, kept there since the sixteenth century and still kept there now.
The castle's rarest thing is easy to miss: a baroque theatre from the 1760s that survives complete, with its original stage machinery, painted flats and props still functioning. Only a couple of theatres in the world are intact to that degree, and it is shown on a limited number of tours to protect it.
The other half of the town is aquatic, and this is the part visitors most often miss. Canoeing and rafting the Vltava is close to a national pastime, and Krumlov is one of the standard put-ins — you can hire a raft cheaply, get shuttled upstream, and spend an afternoon drifting back down through forest and small weirs with the town arriving around the last bend.
Honest note. It is heavily touristed, and in daylight the main square belongs to coach groups from Prague. The counter-move is simple: stay the night. The crowds are day-trippers almost to a person, and the town after six is a different place.
02How to experience it
Come by bus from Prague or České Budějovice and stay over. Walk the town and the castle ramparts late in the day, book the baroque theatre tour in advance if it is running, and give a full afternoon to the river. Eat away from the main square.
From the locals
A Czech contributor to the r/travel country thread ran through the places worth adding to a Prague trip and did not hedge on this one: "Český Krumlov: yes! Very touristy, but rightfully so. It's a fairy-tale-like town."
The more useful comment came from a traveller pushing back on how the town is usually sold: "I've seen quite a few people mention Český Krumlov, but no one seems to have mentioned the reason I think it is so good. Apart from the beautiful town, the second attraction is the [Vltava] river that flows through it — kayaking, canoeing and rafting down it is like the national pastime for the Czechs." Their logistics are worth copying: staying at a hostel in town, "a group of us hired a raft for 10 euros each. None of us had any rafting experience. For that price they also took the raft and us about 10 km up the river."
(That commenter names the river as the Danube; it is the Vltava. Corrected in brackets rather than quietly, because the rest of the account is first-hand and worth keeping.)