I´ve never been to the places already mentioned but if you want a beach holiday but not the Brits then Erdek may be for you.  It´s a tourist area most frequented by locals.  I only spent a day there so I don´t know what hotels are like. There are some photos on TC but I can´t remember the name of the member that posted them.

http://en.wikipedia.org/wiki/Erdek  Location . . . Marmara Region

photos  http://uk.images.search.yahoo.com/search/images;_ylt=A1f4cfiRcMtI7WABnq1LBQx.?ei=UTF-8&p=erdek&fr2=tab-web&fr=slv8-msgr

If it´s not only a beach holiday you want then Iznik may interest you.  It has a huge lake, and it´s  famed for its ceramics, has some lovely old streets and buildings and a museum.

http://www.turizm.net/cities/iznik/  location and information

photos  http://uk.images.search.yahoo.com/search/images;_ylt=A1f4cfk4dMtIEOgAuXtLBQx.?ei=UTF-8&p=iznik&fr2=tab-web&fr=slv8-msgr

These places aren´t too far from Istanbul, relatively speaking.

 I already have an appointment with one of them....

I have posted this link under another thread (Web links about Turkia) . But it might be helpful here to get other destinations´ ideas

http://www.travelturkey2006.com/turkey_map.htm

How about a mud bath in Caunos?

 A mud bath? Yeahhhhhhhh, but Caunos is too far from Turgutreis...