Mardin is a highly addictive and unmissable spot. Minarets emerge from a baked brown labyrinth of meandering lanes, a castle dominates the old city, and stone houses cascade down the hillside above the Mesopotamian plains. As a melting pot of Kurdish, Yezidi, Christian and Syrian cultures, it also has a fascinating cultural mix.
Just don't expect to have the whole place to yourself. With regular flights from İstanbul, you'll see lots of local visitors in summer. The Turkish government also has plans to promote Mardin internationally as an iconic Turkish destination like Ephesus and Pamukkale, and have (optimistically) projected up to five million domestic and overseas visitors annually within ten years.
Mardin's honey-coloured collage of old buildings and markets is still definitely worth a look, and for somewhere extra special, take a detour to Dara or Savur.