I am planning a trip to Turkey. If I only have a week,I'd like to see Istanbul and one other destination. What do you recommend?

Blancalita, there have been several posts on this so if you use the search function for "Turkey" you will likely get a lot of good info.
That said, with one week I decided to stay in Istanbul and found that there were still things that I did not get to see/do.

Seamus is right. You could stay in Istanbul and do some day trips. If you truly want to get to another location, Cappadocia was marvelous. but, you'll really be pressed for time if you chooe to do both.

We spent 2 nights in Izmir (and saw Ephasus), then took a bus to Istanbul, and spent four nights there.

I wouldn't recommend the bus part (I'd fly instead) because of time, but that would work nicely.

After seeing Turkey, I will go back and spend several weeks there if possible so I can see much more. It's a wonderful place -- but it's far-flung and an incredible country to visit, so a week really isn't nearly enough.

Happy travels,

Jules

Hi

Yes another vote for Cappadocia (truly amazing and unique landscape). If you can afford it (I think it costs around €300) take a hot air balloon ride for a true once in a lifetime experience.

One more vote for a week in Istanbul. There are lots of thing you can see in Istanbul. Maybe your next trip you can choose other destinations. Sultanahmet square is really wonderful. Maybe it depends on your budget but you can find tremendous hotels in Sultanahmet.