they have lots of activities, delicious turkish cuisine. most of them are cheap. there are many places for people to relax. i mean they have whatever you need to live or study.
erzurum=konya=kayseri=disappointment for humanity. they are giant conservative, redneck villages (not real cities at all). i suggest 1. antalya 2. bursa 3. samsun or kocaeli 4. samsun or kocaeli for you. they are also boring cities, but there are not many other options.
i have been to these 5 cities, and when i was there, i didn't miss istanbul much; also, bursa was the capital of the ottoman empire. if you are interested in history, you can find some.
compared to others: big city, people are open-minded, lots of options for fun.