Prague spreads the Charles Bridge over the Vltava with Old Town Square's astronomical clock, Prague Castle as the largest ancient castle complex (70,000 sq m), and Český Krumlov's UNESCO old town two hours south.
Prague Spring International Music Festival from May 12, Vltava cruises peak, and Old Town Square terraces pleasant before peak summer crowds.
Prague hits 28°C and Charles Bridge crammed by 10am, but the Letná beer garden above the city stays cool, and Czech beer halls (pivnice) at full operation.
Prague's Letná Park and Petřín Hill chestnuts turning, mid-October weather pleasant for walking, and the Pilsner Fest in early October.
Christmas markets at Old Town Square and Wenceslas Square through December, Prague Castle in winter snow photogenic, and Czech beer halls indoors.