Kadıköy is a wonderful district, it's very diverse and not so packed compared to Istiklal. There are really good local fish bazaars, pubs and vintage shops. The street art scene in Kadıköy-Moda area is very colorful.
There are couple of buildings with tall murals done by famous foreign artists like; Pixel Pancho, Dome and Amose during the Mural Ist Festival 2013. One piece not to miss is just near to the ferry; Misbah Muayyes Street done by local artist Esk Reyn.
There will be another Mural Ist festival, in the next couple of months which will host world wide artists to create remarkable murals in Istanbul.