In Istanbul the metro, tram, bus, ferry and funicular all run on one card: the İstanbulkart. Paying cash is no longer possible on most of them, so this is a first-day job.
Where to buy. From the machines in metro and tram stations, from some kiosks, and at the airport. The machines speak English as well as Turkish. You pay a small amount for the card itself, then load credit onto it.
How much to load. For two people over three days a moderate balance is enough, and you top up at the same machines. Do not overload it — getting money back off a card is a nuisance.
Where it works. Metro (M2 Şişhane is five minutes from us), tram (Karaköy, seven minutes downhill), buses, the public ferries, the Tünel funicular and the Metrobüs. One card can pay for more than one person: tap it twice at the gate.
Transfers are discounted. A second and subsequent journey within two hours costs less, so a route planned with changes is often cheaper than one long ride.
The ferries. Before you buy a Bosphorus cruise, try this: the public ferry from Karaköy to Kadıköy. The same view, paid with the card, and full of people who actually live here. Time it for sunset.
One warning: the airport line (M11) and the HAVAİST buses cost more than the standard fare. The card still works but the balance goes quickly. If you are arriving from the airport, buying and loading the card there is the practical move.