Only one day free in Prague?

You must: Tour the castle. Walk across the Charles Bridge. Wander through the old city to Wenceslas Square, where you have to eat a sausage and drink a Czech beer from a street vendor. Head over to the Astronomical Clock and Staromestke Namesti (old town square). Take in an afternoon concert, perhaps an organ performance at the Jan Huss church. Have goulash for dinner, then an absinthe before attending an evening concert at the Municipal House. Life will be good.