Since everybody here loves creating some sort of crazy touring itenerary when booking awards, Turkish has just made it easier to add another city to your journey without using up a stopover. Passengers of any class transiting internationally on Turkish Airlines can now take advantage of a free tour of Istanbul if their layover is longer than 6 hours between 0900 and 1800 local time. The tour lasts 6 hours and includes food, transportation, and admission costs to any museums. This is in addition to the hotel services that Turkish offers for long layovers. Thus to maximize cities you are interested in, for a trip you could schedule IAD-FRA Stopover FRA-IST Long layover to take advantage of the program, IST-wherever and enjoy your destination. While I know that you can not get the full experience of Istanbul, it is still great for maximizing a trip.
I love this idea, as I really think it will get savvy Star Alliance redeemers to try Turkish just to take advantage of being able to add another city on their travel experience without incurring the penalties of adding another stopover. In fact, this really has me contemplating where I would want to go on a trip where I could pull off the day stop in Istanbul just to experience the city.
(Thanks to Mark Ashley for pointing this out)