Pleases are a Must to Visit

You should take advantage and visit Bursa, that is where the ottoman empire originally started, the place is full of history. You can take the ferry from Istanbul to Bursa.

Hello FeminineHeart, I think you should visit Yerebatan Sarnici, Ortaköy Mosque as well. There are also nice places. You should make a search with "Koru" keyword. "İstanbul'daki korular" and also "Tarihi köşkler İstanbul". You must consider Üsküdar Çamlıca, Beylerbeyi Sarayi, Topkapı Sarayı and many others...

Well outside of Istanbul.. Bursa is one of the important places because it was also a capital of Ottoman Empire before Istanbul. Edirne Selimiye Mosque is also one of the most beautiful architectures of "Mimar Sinan".

That's all for now. I will try to add some more later.

Hope you enjoy your times here.
İyi günler. Hoşça kalın. Bye bye. 

Thank you for all of your suggestions. They were very helpful.

Istanbul is an international city, However there are a lot of historical places such as you have already mentioned above. If you want a piece of advice, "Princes Islands" or in Turkish "Adalar". One of the most beautiful destination. You can reach there by boats, 2 hours of sea voyage is really a worthy adventure . Moreover, you can discover the islands by horses.
Best wishes.