As a couple, St Vitus Cathedral works.
Architecturally the most breathtaking interior in the city — the nave height, the Mucha window, the chapel of St Wenceslas reward anyone who steps inside. The catch: the free zone is a cramped pen near the entrance, so you need the castle circuit ticket to actually walk the nave, and it's invariably packed and echoing. The optional tower climb is 287 steps with no lift.