The Basilica Cistern is one of the few sights in the old city that works well in the middle of the day. It is underground, the temperature stays around fifteen degrees, and the visit takes under an hour.
Built in the sixth century to supply the Great Palace, it holds three hundred and thirty-six columns arranged in twelve rows. Look for the two Medusa heads used as column bases at the far end — one inverted, one sideways, both reused from an older Roman building.
Tickets are sold at the entrance and the queue moves quickly outside peak hours. From Hotel Yasmak Sultan the walk is about ten minutes along the tram line, and the entrance is a minute from Hagia Sophia, so the two fit comfortably into one outing.