The Kariye Museum is a very small former mosque located on the western edges of Istanbul. It is worth a track to see the details of the gilded mosaics and frescos. The beauty of the small space leaves you wondering how spectacular Hagia Sophia must have looked like at its finish.
Chora Church is a Byzantine monument decorated in the 13th century with intricate mosaics. The church also has this serene cupola painted with the Virgin Mary and Jesus in the center surrounded by her ancestors. It's a little out of the way but so worth the effort.