Although one could spend a week in Istanbul, it is also nice to include other parts of the country.
Cappadocia is definitely one of the natural and historic highlights in the country. Three nights in perhaps a cave hotel is ideal for exploring the area. On the way from the airport, there is the Caravanserai located near Avanos that is a good rest stop. This is also where your clients may see The Whirling Dervishes Show. Main attractions in Cappadocia are Pashagabi, Zelve, Goreme with its open air museum, and Pidgeon Valley. There are underground cities to discover, hot air ballooning, horseback riding, and plenty of hiking trails.
Bodrum is a wonderful place for a few days of sun and relaxation. It has a lovely marina and the bay is beautiful. It is often called the "St. Tropez of Turkey." A must-see site here is the Underwater Museum located in the Castle of Bodrum. This is also where your clients can begin their Blue Cruise aboard a gulet.