Both cities are amazing! So much history, culture, fantastic food, etc… make sure to take a boat across the Bosphorous in Istanbul. You can even take a commuter ferry and sit outside (on top) for the amazing views. And make sure to visit a rooftop restaurant/cafe in the Sultanahmet area for the spectacular views (especially around sunset).
I also love Athens for the ancient ruins, museums, outdoor markets, coffee (I’m not a coffee drinker but for some reason, they make delicious coffee concoctions!)