If time permits, definitely check out Bursa, about two hours from Istanbul. It provides a perfect escape! It is much quieter and smaller than the city in which Asia and Europe meet. The old city in particular is peaceful and beautiful; the Green Mosque and Mausoleum are not to be missed.
Insider's tip: if you are looking to buy ceramics, skip Istanbul's Grand Bazaar and shop around the old city of Bursa. We happened upon a small shop that was selling ceramics at half the price of what we saw in Istanbul.