After a super early (5 am!) arrival in Istanbul, we spend the day around the old city. This is one crazy place and you can feel the energy wherever you go.
At the airport, we managed to buy a Turkcell SIM card with enough data to last us for the week. It's definitely cheaper (and quicker) to buy it from shops away from the airport but it's super handy to be able to navigate and access the Internet straight away.
We also purchased a public transportation card (Istanbul Kart) which gives you easy access to all public transportation options around Istanbul. You load credit at kiosks and the card can be shared between multiple users. Istanbul is really congested and during peak hours it's best to make use of the trams and metro trains to get around.