When are you going? Because, if you are American you can get a visa on arrival up through April 9, as of April 10 2014 you will need to secure electromic visas for each and every traveler ($20 each, online, Visa or MasterCard, good for 90 days multiple entry within 180 days of validity).
www.evisa.gov.tr/en/
Now as a traveller I am a slow traveller. Five days in Istanbul, you'll barely see what you can, if you include an all day cruise using the
ferry up the Bosphorus to the Black Sea, with a hike up the hill at Anadolu Kavağı and having lunch at a local fish restaurant or café, maybe a stop at one of the museums and see the old trader's mansions on that route, the defense walls and castles, etc.
See the mandatory" sites, but head to Uskudar and a fish meal, go to a café for some tea or coffee, really let the city begin to sink in- it takes a week, and that might be cutting it short, with jet lag, getting down with the local time, a half day if you wish to really explore and wander the
Grand Baazar and vicinity, etc.
Go to Tom Brosnahan's EXCELLENT website,
Turkey Travel Planner, and see the myriad possibilities. Just look at his Istanbul transport options
here,
If you love Turkey return for another week - in
Cappadocia.