Písek is a small South Bohemian town on the Otava River, best known for a stone bridge that predates Prague's Charles Bridge. It was once a royal city on a trade route to southern Europe, and the old centre still has enough walls, churches, and civic buildings to make a stop feel earned.
The town is not a major destination in the way Český Krumlov or Kutná Hora are, but it passes the spare-day test if you are already crossing South Bohemia. Písek Stone Bridge is the obvious anchor; the Písek Castle and Prácheň Museum and Municipal Power Station add enough local character to round out a half day.
Come for a walk by the river, the old lanes around Velké náměstí, and a meal near the bridge. Písek is best treated as a pleasant, historic pause between Prague, Tábor, and deeper South Bohemia.