Originally Posted by INEC

I've never done the short visit to the city myself, but I think it's doable. I'd suggest leaving your hand luggage in the TK *G lounge. They have lockers. Most places accept credit cards, but cash is always best. The airport has ATMs.

With a 8-hr. layover, I'd leave at least 2hrs. grace to return the airport. So, with 5.5 - 6hrs. to spare, I'd say you have plenty of time to visit various parts of the city for sightseeing and a meal. The museums might be closed, but the architecture, sights, smells, and Bosphorus views are great.
- Taksim, Sultanahmet, Bebek, Ortaky. Lots to see and do. To be safe, plan for 50min each way
- Atakoy. Not much, but some malls, shops and restaurants, right by the marina. Only 15min each way

Stay close to the main public areas and don't let strangers chat you up (like any big city, anywhere) - Enjoy!