Two days in Istanbul is good, get a private tour guide. We also went to Cappadocia for two nights which was outstanding....stayed in a Cave Motel and did a Hot air balloon ride and toured underground cities.

Vienna is not far and there are other sites in Turkey as well. I did my mission in Spain and recently ran the Barcelona marathon (did the Vienna one as well). Spain can be a good trip but more to see around Istanbul.

Just as safe as any Europe location.