20 hours in Istanbul Istanbul Recs 2015
Should also mention Bebek. I don’t know it very well but I’ve heard good things and it’s on this (crazy) list of 50 coolest neighborhoods in the world.
https://www.timeout.com/travel/coolest-neighbourhoods-in-the-world
Probably has a bit of a college town feel with Bogazici University right there, and right along the water.