Although the Church of St. Savior in Chora (Kariye) might be off the beaten track, it is quite a find. This small non-working church, originally built in the 5th century, holds a break taking collection of Byzantine art. Of particular interest are the frescoes and mosaics.
Small entrance fee required. More information is available at the official website: http://english.istanbul.gov.tr/default.aspx?pid=315