Historical subterranean funicular (2nd world's oldest metro). It is cute, has a nice exposition and mockup at the bottom station, and a least tiresome way of getting up to İstiklal ave.
Being the 2nd oldest underground train in the world (after London Undergr.), the funicular between Karaköy+Tünel Square opened in 1875 as a means for traders in the area to commute up the hill. :)