How much Lira for 1 day

Unless you go to a restaurant or tiny shop that don't take credit cards, you probably won't need any cash. If you prebook Hagia Sophia tickets you can use credit card, and I'm pretty sure the cistern takes credit cards too. There is no entry fee for Blue Mosque but check prayer times in advance of your visit so you can plan your time there. In the off chance you need cash, there are plenty of ATMs in the big square near the mosques.

Take note that it is at least a full hour drive from IST airport to Sultanahmet area. I'm a little unclear with "one day" plans - you're arriving when? Leaving when? Your sightseeing time may be very truncated (and depending on the answers, you all may be super jet lagged too).

I'm pretty sure the cistern takes credit cards too

True! I used my credit card for admission last month.

Thanks CL

I've decided to try to see whatever I can during the layover in Istanbul, after a lot of back and forth discussions with family and friends, and as well as input from some of the members in this forum.

The layover in Istanbul is during my return leg to Singapore after a 3 week jaunt in Italy, thus jetlag wouldn't be an issue.

Will be arriving into IST from Naples at 13:40 and will likely be at hotel around 3-4pm. Departing next day to Singapore at 13:45. Well aware that we don't have much time but will make the best out of the situation.

Will be arriving into IST from Naples at 13:40 and will likely be at
hotel around 3-4pm. Departing next day to Singapore at 13:45. Well
aware that we don't have much time but will make the best out of the
situation

Hagia Sophia is open 9 a.m. - 7:30 p.m., you might be able to swing a visit on your arrival day if your flights and transport from airport are timely. The Blue Mosque closes around 6, so depending on prayer times you might be able to get there arrival day or first thing the next day. The cistern is open 9-7.

My last two cents, give yourself a generous amount of time to get back to airport and check in, the security there is very thorough including a security checkpoint right as you enter the airport (separate from security going to the gates see info here). Three hours ahead of an international flight is generally recommended (plus an hour to get to the airport, traffic is simply awful there) which doesn't provide you much activity time on your return flight day as many things aren't open until 9, except possibly Blue Mosque.

Good luck!

In addition to the daytime hours that CL mentions, the Basilica Cistern is also open late into the evening with a ticket that you can only purchase onsite, credit cards accepted. I arrived (in August) at 8:45pm, with no line to buy my ticket and maybe a dozen people visiting.

The Basilica Cistern Museum is open to visitors every day of the week between 09.00 and 23.50.

Remuneration
09.00-18.30 | Ticket: 900 TL
19.30-23.50 | Ticket: 1.400 TL

A note on location: Google Maps took me to the exit for the site. I had to ask a guard to point me to the entry, across Seftali Sk and just around the corner on Yerabatan Cd.

Another later evening option is the small, but beautiful Rustem Pasa Mosque, with lovely blue tiles covering the walls. It is near the Spice Market, open 6am-930pm, except prayer hours.

Last year 58 million people passed through IST Airport’s international Terminal. The total number of passengers was 76 million. IST is now one of the busiest airports in the world. It is gigantic, as it was designed to be the largest and ultimately busiest airport in Europe or Asia.
Because there is so much walking required at IST airport (there are very few moving sidewalks) and there are literally dozens and dozens of baggage carousels to navigate before going through Immigration and customs—- I don’t think you can count on arriving at your hotel before 5 pm (17:00).
Unless you’re arriving on the weekend, you’ll likely be traveling the 30 miles to Sultanahmet at the beginning of the afternoon rush hour by the time you get out of IST—most likely around 3pm.
Note the closure times of the places you would like to visit and realize there will be lines to (1 ) buy admission tickets and (2) to pass through the security machines now required to enter attractions in Istanbul. For a flight home at 13:45 you’ll need to be back at IST by 10:45 which means getting into the taxi headed for the airport by 9:30. ( You actually do need the 3 hours at IST to get through the airport gauntlet because IST screens everyone’s belongings two times at two different security checkpoints).
You are not going to have a lot of time for attractions on the day you arrive in Istanbul- especially with three children. To safely make your flight the next day- I think you just have time to jump in the taxi for the airport.

Hi Kenko. Yup, as mentioned I had lots of back and forth discussions on whether to go for it and just try to make the best of the situation and see whatever we can during our limited time on our layover.

Few things working in our favor to cut the time spent at airport:
- business class flights for both legs
- pre-booked 2-way airport private transfer

I weighed the cost of staying at Yotel vs a hotel in Sultanahmet. Yotel was incredibly expensive. The cost is equal to my Sultanahmet accommodation AND 2-way airport private transfer. Hence why I decided to just go for it and head to the city. At the minimum, we will just spend the evening along Bosphorus and just soak in the atmosphere

Great that you understand the time constraints you have.
Be sure to go to The Suleiman Mosque - the highest point in the old city— for the views across the Golden Horn toward the Galata Tower. It’s a spectacular view of Istanbul, day or night.
Have a great trip!