Barcelona is cool, Sydney is always beautiful, and I liked Istanbul but for me the must see things are usually outside the cities. These were on my list for September '20 and are right up there with Pamukkale and Cappadocia. Interestingly google assures me that Pamukkale is a city, so I guess it gets promoted to the first list.
I have only personally travelled throughout North America and Europe (and mostly on business) so my comments are constrained by those limits. However, my recommendation is to visit two cities on the trip: San Diego, CA and Vancouver, BC. They are both gorgeous harbor cities on the Pacific Ocean and look remarkably alike. The only major difference (beyond climate) is that San Diego has palm trees everywhere and Vancouver is covered in pine trees. Otherwise, you could confuse one harbor with the other. Great places for food, fun, sightseeing, etc.
Been watching these guys lately, really want to visit Barcelona: https://www.youtube.com/c/JamesBlick
I second Barcelona, it's beautiful, even if you're not a fan of large cities like me. Pretty much every city on the coast of the Mediterranean Sea is beautiful. Gothenburg (Sweden) is also nice.
Sintra is a wonderful town. You can visit the old Moorish castle just outside of town, but my favorite thing is visiting the Quinta da Regaleira grounds. It is a stunning place where you can reach by caves, grottoes, and tower bases, with an impressive small network of tunnels. It was one of our trip 's highlights!
Quebec City in Canada is the only fully walled city in North America. The "Bas Ville" is a world heritage site. This part of the city dates from the 1600's and is EXACTLY as it was then. There are cannons everywhere and the regiment dresses up in the old blue and white and carry muskets from the old times. Then you have the "Carnival" which is a sort of Mardi Gras in the winter. People from all over the world go to be a part of that. The Chateau Frontenac in Quebec City is one of the most impressive buildings in the world, truly something to see!
Venice is an amazing walking museum and being at the Carnevale is like being in a Fellini film, extraordinary! And the gastronomy is to die for...