If you want to visit Istanbul, probably
you already know some place to see, but I will try to propose some "maybe"
not known destination.
For example the district of "Kadıköy"
which is on the Asian side. You can arrive there by ship from the districts of
Karaköy, Eminönü or Besiktas. When you are passing to the Asian side, you will
see the "Haydarpasa" train