Haven't been to Istanbul, but in other more touristy places in Turkey it's fine to be a single female traveller. Western/asian (clearly not turkish women) do attract attention, but in my experience the harmless type.

I am sure that in Istanbul like other parts of our world there are places which are safer than others. Do a google search on it

If you don't mind cold, then the northern lights in Norway are quite a spectacular sight.

To prevent loneliness I would book one or two tours to whatever museum/sights/destination you want to go. You'll meet other travellers and I've often found myself in the company of the same people several times during my trip. Such day trips (or sometimes two days or more) are perfect for when you want to meet and interact with someone without being stuck with them for the duration of your trip.

But the world is a wonderful place, so narrowing it down to what you like, then searching for destinations offering that sounds like a good idea.