DH turns 60 in 2020. DC are old enough to be left at home/uni (I'm talking 18+!). We're thinking June.
Both appeal to us. Personally I prefer Istanbul because, bluntly, given the direction the country is heading in, it may not be accessible to us in the near future; but I accept it comes with a bit of risk. (DH never saw Cairo before it became 'risky'; I was fortunate to go as a backpacker in the 80s). And it will be DH's and my's decision, not dragging DC along.
TravelMan has raised the possibility of Krakow which looks interesting, but I hope our ability to visit there will remain in the future!
I, for example, regret not having been to Jerusalem/The Holy Land, although everything I read makes me feel it might be disappointing, but that's not on the cards right now.
Be aware, we aren't scaredy cats; DH travelled through S America, including Colombia and Venezuela, alone, in the 80s! I backpacked through SE Asia around the same time. But places that were open to us then, aren't now. And we're hitting 60.
So would you 'go safe' (and possibly a tiny bit pedestrian) and go Krakow, or 'do' (I hate that term!) Istanbul while we can?