Even on a tight schedule, Municipal House (Obecní dům) earns the hours.
Allow 1–1.5 hours.
If you love Art Nouveau, the Smetana Hall, mosaics and Mucha's Mayor's Hall are the most ravishing interiors in the city, best seen on a guided tour or by attending a concert. The catch: the lavish rooms are tour-only on a set schedule, so you can't just wander in; the café and exterior are free but the real splendour is behind the ticket.