I have a long layover in Istanbul and am interested to see a part of the city. A couple questions - will I need a Visa? If so, can I purchase at the airport or do/should I apply for an e-Visa? How long to town? Should I use the tram or go by taxi? What sights to see - Blue Mosque, of course. What else? Finally, would it be best to set up a tour with a local travel guide who could provide transportation and travel to sights? Open to suggestions.
Yes, get the eVisa online, it will save your time. Are you going to have your luggage with you? How much time will you have in total on the ground (allow at least 1 hour to get to the airport each way = 2 hours just in transit)? I used the tram to get from the airport to the Old City (Sultanahmet stop), it was easy and cheap. The traffic in Istanbul can be insane (in peak periods) so the tram is more efficient. I don't think you need a guide to see the sites, but it would help immensely to have a guidebook (the RS guidebook is quite good, or at least look up what you want to see on TripAdvisor). Many sites are concentrated in a not-too-big area and you'll probably only have a chance to see a limited few. Topkapi Palace was my favorite but I spent all day there. In the Old City, there's the Blue Mosque, Haghia Sofiya, Archeological Museum, and lots of other sites (including some small ones that hardly anyone visits but are super interesting).
You didn't say how long your layover is, but depending on the amount of time available I'd do the Blue Mosque first, followed by Topkapi Palace, and then Haghia Sofiya .
If you go to the Blue Mosque, you will be in the heart of the old city with many sights within walking distance. The Rick Steve’s guidebook can give you plenty of info to help you plan.
If you are flying on Turkish Air, they offer FREE tours for people who have layovers of over six hours, depending on whether your arrival and departure times work with their scheduled tours. Here is the info:
Scroll down to the very bottom of the page to view their different options. The tours pick you up at the airport and return you there, taking some of the stress out of the day.
Another possiblity would be to hire a private tour guide. Here is an agency that might help you. They have worked closely with Rick Steves over the years and use some of the same guides:
Istanbul is an amazing city. You will want to return! Have fun.
Another vote for Topkapi as #1.
Just a note on Topkapi Palace:
I was there a year and a half ago and quite a bit of it was closed off being renovated. It seemed like I saw one third of it, for the same price as a previous trip when I saw the whole palace. I would definitely check to see how much will be available to tourists.
It is truly wonderful, but if still under renovation, I'd suggest another using your time elsewhere. Tourism is way down so you'd probably get into the Blue Mosque without much wait time. We got in right away in the summer of 2016 as opposed to three years earlier when the line went all the way down the Hippodrome.
If I had one day I'd see Hagia Sophia. Most spectacular piece of architecture I've ever seen. Then simply roam about the bazaar.