Hi there. I'm going to Istanbul with my husband at the end of the month for 6 days and we usually find it quite difficult to be in a busy city for that many days. Any suggestions for trips outside the city, for 1-2 days?

I did a wonderful day trip by boat to the Princess Islands (? so long ago i forgot the name)m - but some islands about an hour away that were lovely. Do some research into that.

Or you could take a bus to Ephesus, on the coast not too far from Istanbul.

A second vote to Princes´Island. No cars allowed, just bikes and horse carts. If I remember right the boat leaves near the Galata Bridge.

In the event you opt to go to Ephesus (I recommend it highly), suggest you go by plane. You can take an early flight to Ismir, have aguide and driver pick you up and spend the day at Ephesus. In the event you still have the drive, you can stay overnight and explore the surrounding area. A good break from Istanbul.
I believe that if you are flying Turkish Air from the US, you can add on these flights at a very reasonable cost ( at least that was true a few years back). An alternative to a private guide and driver would be a packaged 2 day tour which is probably offered by most tour companies servicing Turkey.

You can visit the islands, also my Vote. For a 2 day break you can consider Cappadocia or Ephesus as well,
There are frequent flights for both destinationa.
Happy travelling,
Murat

Princess Islands could be a wonderful chose. Many of my tourist friends love it.