Hi Ali
I think Istanbul is a great choice. It's an amazing city with so much to see, including the bazaars, Hagia Sophia and the Blue Mosque. There's so much history everywhere you go but it's also a very modern city that is keen to be part of Europe, so you and your group will have plenty to talk about every day.
Also, as you haven't travelled in a group before, you might find five days is a good time to be away.
I'm sure you will enjoy this break.
Kind regards,
Olly, your editor